If you're coming on for 20-30 mins vs tired legs with a place to earn in the team against fodder, then yeah .. you have a very, very good chance of scoring. More than if you played the full 90? Probably not, but I'd not be surprised to see it's close. Again, Muriel last season scored an insane amount in these circumstances .. as did guys like Ole, Chicharito etc. They were specialists as their skillset suited that.
Both appearances & minutes per goal would be flawed in this case. I remember Vardy playing for England though, and he just looked a poor fit, and I'm sure anyone else on here who remembers it would say the same. Didn't get in to games at all. He may well have ended up looking comfortable, who knows?
But regardless of any of that, Vardy is a top striker, and extremely underrated. It may surprise you when I say if we were heading in to a CL final tomorrow, or if England were in a world cup final, I'd go with Vardy over Kane. His record is objectively better against the top sides as he's superb at beating a high line, and a constant handful for defenders.