My wife recently got a VP9. We compared it to a PPQ when she got it, and the 320. The 320 "felt" bigger. We were at Bass Pro and couldn't try the triggers, so I can't compare those, but just feeling the gun in the hand the VP9 and PPQ were so close, it was hard to tell which was which, but fit like the proverbial glove. 320 felt good, but not as good.
At the range, VP9 has the best tactical trigger I've ever shot in a striker fired gun. We tested it next to a PPQ a guy had there, and it wasn't as light as the PPQ, but has just as good a break and a better reset. The PPQ has a break that way back, so very little over travel, and so light it wouldn't be a good carry firearm. The VP9 is probably the ultimate stock carry gun, with the exception of only 15 rounds rather than 17 in the mags.
Compared to the G19 (which my wife also has), the VP9 will simply outshoot it stock. It's sighted in at the factory, and with the clean trigger it's just easier to hit on target. Also, fits better in the hand. It's like the difference between a Jeep Cherokee and a Porche Cayenne, without the price difference.
Don't mean to derail this thread on the 320, I'm sure it's nice, but these 4 guns seems to come up in conversation a lot, and the 320 is the only one I've not shot, so can't comment other than to say it's somewhere between the G19/17 and the VP9/PPQ in terms of how it feels in the hand, and I would guess in terms of the trigger as well, from other comments I've heard about it.