It is now common, at least among pastors and pastoral theologians, to speak of marriage as a vocation. But it wasn't until at least the mid-20th-century, around the time of Vatican II, that such language came to be widely used. Indeed, I can find hardly any use of it between New Testament times and the last fifty years or so. And for more than one good reason, IMHO. Which is why I find the newfangled language rather odd.
For one thing, marriage is what most people are supposed to go in for. So it doesn't seem that there's anything there to be "called" out of in order to serve God. If a vocation is a call from God to serve him and him alone, fully and explicitly, then marriage can't be called a vocation in that sense. For as the Apostle says:
The unmarried man is anxious about the things of the Lord, how to please the Lord. But the married man is anxious about worldly things, how to please his wife, and his interests are divided. And the unmarried or betrothed woman is anxious about the things of the Lord, how to be holy in body and spirit. But the married woman is anxious about worldly things, how to please her husband. I say this for your own benefit, not to lay any restraint upon you, but to promote good order and to secure your undivided devotion to the Lord.
But of course that doesn't settle it. Paul wrote as he did to the Corinthians at a time when the Lord's return in glory was expected within the lifetimes of the believers being addressed. The resulting sense of eschatological urgency made it natural to recommend that one not marry if one was not already married, so that one could devote oneself exclusively to the spread of the Gospel. But some Christians—many in fact—were and are married. That's why Paul also wrote to the same believers: "Only let each person lead the life that the Lord has assigned to him, and to which God has called him." Thus the question whether marriage is truly a "vocation" remains open from a scriptural standpoint, so that we should ponder the question from another angle.
In the same letter, and purely in the spirit of pastoral advice, Paul tells "the unmarried" and "widows" that "it is better to marry than to burn." The question whether widows should remarry appeared to be a rather significant one for the early Church, for male life expectancy at the time tended to leave many rather young widows in its wake. Paul gave what appears to be a natural answer: if a given widow's sexual needs are such that she is unable to remain continent, then she should remarry rather than fornicate. And we're not just talking widows here, for he also says: "... because of the temptation to immorality, each man should have his own wife and each woman her own husband" (emphasis added). Now if we content ourselves with saying the primary point of marriage is to avoid sexual immorality, then marriage seems to consist more in spiritual prophylaxis than vocation. But of course Paul didn't content himself with that, and neither has the Church.
Clearly, spiritual prophylaxis is one point of marriage, and the Church has always presented it as a "secondary end" of marriage. Under St. Augustine's influence, it was of course long taught that the "primary" end of marriage is procreation, which is what justified the self-indulgence entailed by sex. Whether or not sex needs such justification, it is obviously true from a purely natural point of view that the primary end of marriage is procreation. But there is no such thing as pure nature, and marriage is in fact a sacrament; so the old language of primary and secondary ends isn't quite adequate.
Indeed, if we take seriously what Paul says in Ephesians 5 about marriage, we cannot but see the primary goal of marriage as that of being an icon of the mystery of Christ's relationship with the Church—one that is sacramental because it helps to bring about what it signifies. Procreation does not so much compete with such a goal as get subsumed under it. Just as giving new and eternal life to his people is why Christ came, so the transmission of life in the form of children is a spiritually momentous consequence of imaging, in marriage, his relationship with his people. From that point of view, it seems that marriage and family do indeed constitute a vocation. Such is how most Catholic clergy, and not a few theologically educated Catholic laity, now talk.
But there remains a difficulty, closely related to what I call "the paradox of specialness": if everybody is special, then nobody is. In a sense, every Christian is special just in virtue of their baptismal vocation. They are "called" out of the world, of this age, to bear Christ into the world as part of his Mystical Body, the Church, and thus begin their journey into eternal life. In that sense, every Christian has a vocation, and every other "vocation" is but a specification of the baptismal vocation. But if we're all special in relation to "the world," we're not all special in relation to each other. Most Christians are not called either to priesthood or to "consecrated" celibacy, i.e. celibacy for the sake of the Kingdom. The bulk are meant to marry, and some are meant neither for marriage nor for consecrated celibacy. And so it seems best, when speaking of 'vocation' from within the Church, to reserve the term for those called, within the Church, to a special form of service to the Church. Otherwise we can't distinguish what's special about the baptismal vocation from what's special about certain specifications of it.
I'm all the more convinced of that because of what I've heard many priests, celibate or married, and many "religious," all celibate, say about their vocations. Whether or not they started out "wanting" to be priests or religious, they all eventually got the sense that such was what God was calling them to be. Many priests in particular will tell you that they resisted such a vocation at first, taking a long time to surrender to God and finally to answer his call. Of course some always "knew" they would be priests. But the point is that becoming a priest or religious, whether or not one finds the idea initially attractive, is less a matter of what one wants or comes to want than a matter of coming to recognize and accept what God wants from oneself. And that, it seems to me, is essential to the idea of vocation within the Church.
Now with a few rather amazing exceptions, I don't think people usually approach marriage like that—and when they do, there's usually something wrong. Marriage is not the sort of thing one is likely to succeed at if one goes in for it primarily because one believes God wants one to. Sacramental marriage entails a wholehearted gift of self to another in both sexual intercourse and in life as a whole. Accordingly, if one does not sincerely and wholeheartedly desire to do that, and desire it for its own sake as a way of being one with one's beloved, then one is not really going in for it and is unlikely to make it work. I am convinced that such is one reason why there are so many failed marriages among Catholics today. Now that there are far fewer social and economic impediments to divorce than there once were, the sorts of factors that used to keep people in marriages where they never made a true gift of themselves to their spouses have dissipated— and we're seeing the consequences. That should teach us that the desire for marriage has to be originally and primarily autonomous rather than heteronomous: one has to want to marry, and marry this person, wholeheartedly and for their own sake, else one isn't going to make a truly sacramental go of it. And that makes marriage unlike priesthood or religious life, where the vocation comes primarily from above, from God, and must be recognized and confirmed as such in order to be authentic. One can succeed in marriage only if it's primarily something one wants, even if God wants it too; whereas one can succeed at priesthood or religious life only if it's primarily something God wants, and is seen to be such, even if one wants it too. For clarity's sake, I think we should reserve the term 'vocation' for the latter.
So here's what I'd say about marriage as "vocation". Sacramental marriage is a noble specification of the baptismal vocation, and as such is one way of living the Christian vocation. But it is not a vocation in the sense that one is "called" to it out of the normal course of things. Vocation in that sense is had only by those within the Church who are called out of the normal course of things to devote themselves exclusively to the Kingdom by serving the Church as a whole. We need the latter more than ever today.