I know that Vardy's declared himself "fit" and that he's back to training with the rest of the squad, but I'd be truly surprised if Southgate uses him at all. If you're Southgate, would you really risk subbing in a player who's already pulled up lame and was unable to participate in the PKs just in the past week? If I were Southgate, I wouldn't (but then again, I'm not Gareth Southgate).
I think Vardy only gets in against Croatia (or France, if England wins) if there's a sudden catastrophic rash of injuries to Kane, Sterling, Rashford, and Danny Welbeck. At this point, Welbeck has to be considered higher up in the striker pecking order than Vardy.
Of course, I could be completely wrong: in a Hollywood-like scenario, Vardy scores the tying/winning goal against Croatia and/or France, and becomes Sir Jamie...