cart before the horse, man.
how will you use a vacuum sync if you cant get your bike to idle?
even with a bad bench sync, your bike will idle.
a vacuum sync device will not help your bike to idle well if your air fuel mixture is not close to being correct.
if you bike wont idle right now, your air fuel mixture is still off. (pilot jets and mixture screws)
needle clip position can change and your bike will still idle.
maybe this visual will help wrap your brain around it.