Would the fact that a guy has had a vasectomy be a huge turn-off to potential girlfriends?

Ideally you are looking for a girlfriend who has no children and doesn't want any children either. Personally I think this needs to be decision made between you and the person you intend sharing the rest of your life with. Although it is your body, the result is something that will ultimately affect the two of you. I know at 26 you feel fairly certain that you don't want a family, my husband felt the same at 26. At 35 he became a father for the first time (by choice and at his suggestion) and loves it, so don't cut (pardon the pun) your options now, wait and see and if you and your wife still feel the same when you are in your late 30's then go for it.