I have heard people mix vocals on beat and key, but it just does not ever feel right to me, its like two people speaking to you at the same speed and key, how can you respond, similarly, how can you dance? So I stick to long mixing, but I make sure the vocals do not clash, practice getting it so the vocals start on track b when track a is just about to start its vocals up and then close or filter out channel a before its vocals spill over. Or if you want to get safe with it, auto loop the beat before vocals and only cut it out after the other songs vocals have started and it sounds right. Also, beat match, phrase match, guaranteed if you do that on point as well as have good song selection quality and professionally you can do much better than otherwise, and look back proudly at your work, and pass on that passion when you pitch clients, I have booked many events on my confident pitches alone.