I tried again using Ina Garten's recipe:
So creamy and delicious, better than 99% of what's available store-bought unless you can find Calabro (usually in Italian delis). I'm probably just going to make my own from now on.
It worked! But, I read all the comments here and on other sites and after adding the vinegar, I waited (off heat) for 20 minutes instead of 5.
Smitten Kitchen uses lemon juice instead of white wine vinegar; I might give that one a try.