Clients don't really care what tools I use. I just tell them what I'm going to need to provide the guarantees we both signed in the contract.

Other developers can be trickier, but it's very simple to get logical people on the path of version control if you override their changes once or twice. (usually unintentionally)

Actually, the only time I've encountered zero version control, I simply refused to start working until git was introduced into the project.