I very often get requests from people asking to do vectors (and occasionally other stuff) based off my line art.
My general policy is this: As long as you give credit and link back to my work in your description, I don't mind at all. In fact, I'm quite flattered. I also would appreciate that you make me aware that you've done a vector of my work--just shoot me a link or something. Though I may or may not reply, I do appreciate it just the same. Keep those rules in mind and we'll be golden.