Were you riding the slide catch down, causing the slide not to lock back after empty?
That was something I did with my Glocks occasionally when I used the Glock extended slide stop. Haven't had that happen since I switched to the Vickers/TD slide stop. But I can certainly appreciate how that is something that one would get irritated about or even cause to not favor a particular gun. However, seems like you shot the best with the VP9, so in my mind that counts for quite a bit. One of my buddies shoots his Sig P-2202 (or something like that) really well, much better than any of his guns, and he would side the slide stop and it would cause malfunctions. He adjusted his grip such that his right hand thumb was sort of placed on the outside of the base of the thumb on his left hand and that seemed to work for him. Personally, I wouldn't want to adjust my grip for a specific gun.
Funny you mention the Sig trigger. That was one aspect that I've heard great things about the P320. Well, the VP9 as well. I know the trigger on my PPQ is awesome. Something about now that there's more competition with polymer guns and the trigger mechanisms are all similar (or maybe about the same), that that was one way companies tried to differentiate themselves. That said, what is a good trigger can be quite subjective.
I would say the P320 trigger is unique. I've personally never found another that is like it, but then I don't have every gun ever made!
It has almost zero travel. When I took it to Front Sight, one exercise was to fire 2 rounds, do a tactical reload, bring the gun back on target and take up the slack, then decide to not shoot and go back to ready position. Not once, but TWICE I fired when taking up the slack!! The is none -- zero -- nada. I need more time to practice with it so I don't treat it like a standard Glock trigger.
On a side note, we were at 15 yards, and the first time I did it, one of the students yelled, "Did he at least hit the target?" The instructor called back, "He nailed it!" I hit the target near dead center both times!
After running nearly 1000 rds through it in 4 days without cleaning, I'd give it 5 stars!