Trouble is you need a several days (maybe a week or more) and/or a few ejaculations to actually be sterile after a vasectomy... So this isn't the kind of thing that you can flip on in the moment and you're safe.
Better to keep it off until you and your wife are trying for a kid than keep it on and flip it off for saf(er) sex. Why keep your cum on while you're just walking around?