You don't need to remember very much about them -- I remembered basically nothing at all and I was fine, he reminds you of the points of interest in the text. It's just that if you had never read one, I think you'd be thrown for a loop when suddenly there's this big style change from hard-boiled assassin first-person to pseudo-Dumas pastiche.

I think you might be okay with having skipped the last Vlad book; there are a couple of spoilers for the previous ones, but couched in such a way that you would probably not notice :)