You know, I don't know if it's just me imagining things but every since I started using FermCap S, I noticed that the head retention of my beers has improved a lot. I haven't done any side by side brews to compare and prove it, though.
I agree with everybody else, I just use enough vodka to cover whatever I'm using. Most of the time I only use like 1-2 oz max, so 2 cups is way too much in my opinion.
Once alcohol is present, it messes up with the readings of the refractometer and you're supposed to do a correction. You can find them if you google them, but I've never used them and instead use a hydrometer. I use a refractometer just to check my pre-boil and boil gravities just to make sure...
I used to leave my beers in the fermenter for 3 weeks back in the days. Then I started taking gravity readings more often and realized that some beers are done fermenting in less than a week sometimes. So now after they're done fermenting I only leave them in the fermenter for a couple of days...