IT depends on the situation, if you know you can burst someone for example, say your fed with vayne or it's late game(same thing honestly) then your probably going to Q into them to initiate and get in range instead of finish them off. If your not fed and you pretty much playing like a normal ADC should, then AA+AA+Q is better because if they run away you can dash towards them, reposition for your E, if they keep chasing you, dash away and also the surprise burst factor due to so many procs at once.
Not to mention two auto attacks+the MAJOR empowered auto you'll get from tumble+third passive proc and thunderlords makes for bursting someone easy, especially if their bellow 75%. In 1v1 senario's or laning i'd use AA+AA+Q but in teamfights i imagine i'll be using Q+AA+AA or a combination of both more. Not to mention if you use your tumble for the last proc and to reposition yourself and land an E as well, you get another free proc soon after because they'll be stunned for at least 2 auto's. In fact most of the time when i see "CRAZY VAYNE OUTPLAYS" it's the same pattern, someones chasing me? Oh, well Kite+Kite+Q+E+AA+AA+Q dead, and that will usually repeat for every outplay.