There is the "single" pseudo-vocation for both men and women -- but it must be a life like Seraphina (above) described: full of self-sacrifice of some sort.
Mothers and fathers make umpteen sacrifices daily for their children.
The religious life? Need I even mention the sacrifices that nuns, brothers, and monks make for God, on a daily basis?
The priestly life? Ditto.
Strictly speaking, a vocation is a calling from God out of the world to a religious/priestly vocation. The other vocations are not vocations in the strict sense.
However, most of mankind is called to be married.
But for those who can't find a suitable spouse, perhaps God is calling you to a special (exception to the rule) single "vocation". Look for some way to sanctify your own soul, help souls, help the Church, practice self-sacrifice and mortification, and do something that would make sense to fill in this sentence: "God created me so that I could _____". Serve God in the religious life? Yup. Help God in the work of creating and raising up souls for heaven? Yes. Be a middle manager for XYZ company? I doubt it. Help special needs children? There you go!
Single people have the ability/temptation to do what they want 24/7, march to the tune of their own drummer, and in general be completely selfish like no other class of person. That is why you need to start a company, group, apostolate, etc. based on your abilities, sex, and circuмstances. Thomas A. Nelson never married, but look at what he did with the years God gave him (he's still alive, BTW) -- Tan Books and Publishers! A women probably wouldn't do that, but she could use her feminine abilities, especially patience and abilities with children, to help souls in some way. She could study apologetics and try to convert souls, help new converts to Tradition to feel welcome with her friendliness and good advice... she could make chapel veils, scapulars, distribute apologetics materials, promote the message of Fatima, promote Traditional Catholicism, the list is endless. Some apostolates would be better for young women, others would be better for older women (or widows). God has a plan and a place for everyone.
But they all have something in common: mortification and work. You can't just spend your days in your man cave (or in front of the TV eating bon-bons) and have fun, and expect Heaven at the end of it.