For me, it is about getting the right acidity along with the right temp. If I were you, I wouldn't add a certain amount of vinegar just because a recipe calls for it. The recipes assume a certain whey acidity level, and yours might be different. It helps if you know how acidic your whey is to start with.
First off, it is way easier if you add some milk to the whey. Heat your liquid and hold it for a minute. If you don't have any precipitation, add about half the amount of acid that your recipe calls for. Wait for a minute and check if you have precipitation. If not, slowly add more acid till you see curds precipitating. Sometimes if you add too much acid, you will see your curds sink to the bottom instead of floating.
Ricotta made from different cheeses will require different amount of acid. Also, if you let the whey sit around, it will require different amounts of acid. To start with, try making ricotta with whey from the same cheese. recod everything you do till you get a feel for how much acid you need.