The Papal Document, Christifideles Laici, is translated “**Vocation **and Mission of The Laity”
The single life is generally not spoken of as a capital V Vocation because it is not a sacramental end state. A single person may, at any time, feel called to the religious life or to marriage, and act on it. So “singleness” is more of a state in life, than an end-state vocation.
The lay state of life is a state of life just as the various consecrated lives are states in life as is Holy Orders and marriage. Single life is the lay celibate state of life. Consecrated life is not a Sacramental state of life - only Holy Orders and Marriage are sacramental.
Vita Consecrata “The Consecrated Life”: “The equal dignity of all members of the Church is the work of the Spirit, is rooted in Baptism and Confirmation and is strengthened by the Eucharist. But diversity is also a work of the Spirit. It is he who establishes the Church as an organic communion in the diversity of vocations, charisms and ministries.
The vocations to the lay life, to the ordained ministry and to the consecrated life”
Lumen Gentium “Dogmatic Constitution on The Church”: "in so doing, they stand as the witnesses and cooperators in the fruitfulness of Holy Mother Church; by such lives, they are a sign and a participation in that very love, with which Christ loved His Bride and for which He delivered Himself up for her.(11*) A like example, but one given in a different way, is that offered by widows and single people, who are able to make great contributions toward holiness and apostolic endeavor in the Church…
…throughout history the Church has fostered a celibate life in the lay state. Towering among the means of sanctity available to the laity, declared the Second Vatican Council, “is that precious gift of divine grace given to some by the Father to devote themselves to God alone more easily with an undivided heart in virginity or celibacy. This perfect continence for love of the kingdom of heaven has always been held in high esteem by the Church as a sign and stimulus of love, and as a singular source of spiritual fertility in the world”
The Church nor Catholic Theology has never used the Big V and small v distinction anywhere at all. It is something that has crept into Catholic cultural consciousness from somewhere or other.
Of course it is true that many people called to marriage may not find an appropriate spouse, and people called to religious life may have issues that prevent them achieving it. Others perhaps feel no compelling calling and remain single.
That doesn’t make single people any less worthy in the eyes of God, or less a part of the Church.
The above is inaccurate in that God could invite a person to remain in the lay state of life in the secular state and temporal order. Every single baptised person has a vocation and calling since baptism itself is a call from God to holiness and to The Gospel.
I certainly did feel a “compelling call” to remain single living a specific way of life. Back then over 40 years ago, I found this quite confusing; however, research with two priest theologians informed me that it could be a call to a vocation from God. In 2014 my Archbishop gave permission for a Home Mass for me to renew life private vows. I had already lived the specific way of life for over 30 years under private vows.
It’s simply that a single person who meets Mr. or Ms. right, and choose to marry, or feels called and joins the priesthood or religious life is not abandoning a vocation in the way a married person or religious who leaves their state of life is.
The above does not make sense. The Church provides for those in all the various vocations to leave that vocation quite legitimately. I think the only exception might be consecrated virgins, although I am unsure on that point. With the Sacrament of Marriage, The Church does not recognise divorce, however She does recognise legitimate separation under certain circumstances. There is also of course annulment for certain marriages only. Annulment means there was no valid marriage in the first place.
Since “all things are possible to God”, I do not think it inconceivable that God could call a person to a certain state in life for a limited period, while the person at the time may not realise this.