I only ever use lemon, not vinagar when I make Ricotta. I usually have about 10 litres of whey in the vat and cut 5 or 6 lemons in half which I then squeeze one half at a time through a sieve to catch the pips till I see a nice cloudy sort of coagulation taking place in the 85'C whey. Then one more half for luck and leave it to stand for a bit.
The amount of lemon juice that is needed to obtain the coagulation seems to vary slightly I find, perhaps at various times of the year or from different trees the acidity varies. Anyway, the taste of the finished product is really nice.