This is all I've ever been able to find on it.
As you can see, according to the Magisterial documents, single life is NOT a vocation.
In fact, the Magisterial documents don't really talk about single life at all.
It's not even listed as a state in life.
Apparently, adult human life is always meant to be lived in some kind of community under some kind of vow.
Indeed, according to the documents, living without this community life is considered a poverty.
I know this isn't fun to hear for a lot of people, and I'd be happy to hear that there are some references I've missed which flesh this out better, but this is all I've been able to find.
Catechism of the Catholic Church
1658 We must also remember the great number of single persons who, because of the particular circumstances in which they have to live - often not of their choosing - are especially close to Jesus' heart and therefore deserve the special affection and active solicitude of the Church, especially of pastors. Many remain without a human family often due to conditions of poverty. Some live their situation in the spirit of the Beatitudes, serving God and neighbor in exemplary fashion. The doors of homes, the "domestic churches," and of the great family which is the Church must be open to all of them. "No one is without a family in this world: the Church is a home and family for everyone, especially those who 'labor and are heavy laden.'"
2349 "People should cultivate [chastity] in the way that is suited to their state of life. Some profess virginity or consecrated celibacy which enables them to give themselves to God alone with an undivided heart in a remarkable manner. Others live in the way prescribed for all by the moral law, whether they are married or single." Married people are called to live conjugal chastity; others practice chastity in continence:
- There are three forms of the virtue of chastity: the first is that of spouses, the second that of widows, and the third that of virgins. We do not praise any one of them to the exclusion of the others. . . . This is what makes for the richness of the discipline of the Church.POST-SYNODAL APOSTOLIC EXHORTATION,
"ON THE VOCATION AND THE MISSION OF THE LAY FAITHFUL IN THE CHURCH AND IN THE WORLD"
The Various Vocations in the Lay State
56. ... Along the same line the Second Vatican Council states: "This lay spirituality should take its particular character from the circumstances of one's state in life (married and familylife, celibacy, widowhood), from one's state of health and from one's professional and social activity. All should not cease to develop earnestly the qualities and talents bestowed on them in accord with these conditions of life and should make use of the gifts which they have received from the Holy Spirit"(208)