So far as I know, there is no official definition from the Church to that question. There is also considerable disagreement in the Church today on the issue. Some say yes, others say no. So, it is impossible to give a definitive answer to your question.
Were you to ask my opinion, though, my opinion would be that it seems possible that the single life could be a vocation. God calls us, no matter our state in life, to sanctify that state of life in which we find ourselves by achieving holiness through his grace. Although that call to holiness can be perfected in the vocations of the priesthood, religious life, and marriage, our sanctification is not dependent upon entering one of those states of life. In other words, the call to holiness is issued to every human being but calls to the priesthood, religious life, and marriage are not.
Also, some duties in the world stand in tension with the traditional vocations. For example, it is very difficult for a doctor who works hundred-hour weeks (as some doctors have been known to do) to be a spouse and/or parent. Likewise, a cloistered nun would not make a very good politician, and a politician sometimes finds it difficult to maintain a healthy family life. For those in professions that require them to devote an extraordinary amount of time to their jobs, the single life may be the best state of life in which they can achieve sanctification.
For these reasons, the idea that the single life is a vocation appears reasonable. But the Church has not definitively ruled on it, one way or the other.
