The idea of Vasek playing seems incredulous. He's been out with an ailment whose main symptom is sucking your energy away and they play out of 5 sets in DC. I suspect Peter will get called in in the end. I agree with 156mphserve that doubles is the key and if Adil is an improvement for the doubles, he should be picked. (I'm backtracking on my 'singles insurance' because realistically Spain will take the No. 2 singles no matter who plays.) Milos' lack of a backhand volley makes him quite suspect for the doubles and Frank hasn't been playing doubles. In the end, I think I'd have Frank play it though - he's a fairly adept natural volleyer and Milos needs to be as rested as possible for his singles rubbers.
Man, Spain is down to their sixth and seventh picks and they are still guys in the top 60 - staggering depth. (As long as DC comes right after GS's, there is going to be this ongong trouble getting the best players to play.)
They won't need Peter, they have Frank, Frank and Milos can play the 5 setters, if there is a 3 setter then maybe Vasek will play it, maybe Vasek is better now, we don't know, Vasek's 1st match of the season certainly can't be a 5 setter vs. Nico, he's not ready for such a high level match yet, we would for sure lose the tie if that happened, Frank certainly gives us more of a chance right now.