A vasectomy is usually covered 100%. That is pretty much standard because getting a vasectomy is very good business practice for an insurance company. This is true because a vasectomy cost under $1000. The cost of a delivery room alone is more than $10,000.
Being single, they will not benefit from paying for your vasectomy. If you knock up some girl you met in a bar, her insurance has to pay for the delivery of the baby, not yours. That does not mean that they won't cover your vasectomy, but it does mean that they will have less motivation to do so.
I know a lot of men that had vasectomies in thier 20's, with no children, and many were not even married. Doing so is called being "Child-Free". Yes, there is a name for it. We also have "Child-Free" social groups where you can meet other people that don't want children.
Some insurance companies might not pay for it if they know you have no children. Some doctors won't do it for the same reason. This is not a law, it is just a policy made up by someone that thinks he or she knows better than you do. You may have to lie, telling them you have two children by a previous relationship or something like that.
I had mine done when I was 21. I am now almost 42, and very happy with my choice. The novelty of a vasectomy never ends.
Good luck, and congratulations on choosing to be "Child-Free". You have made a wise choice in an overpopulated world. Please spread the word to your friend’s children, so they will do the same!