Cup your hand to choke it out. It usually takes a me a couple of hand position adjustments. It will choke out and die with or without a vacuum leak. The difference is if there is a leak, there will be an very brief increase in rpm before it dies. It is subtle and pretty quick and you'll need to be listening closely.

Engine has to be fully warmed up. If it dies instantly. No leak and you can move on to the next thing.

I've always been able to identify if there is a leak this way, but not necessarily able to find it with soapy water, or spraying stuff around.

I don't know if this is your problem. As I said, I don't think so, but I'm still not quite following your description. If your idle is what I call "wondering" at idle, I have found that it is either a vacuum leak, or the mechanical advance weights are too week and the timing moves around at idle, especially when you put it in gear.

Good luck.