I've had the V3 for several months now, and everything has pretty much been said here already. However, one extra thing I like about the threaded upper collar on the V3 is that I can un-thread it snugly against the bottom of my tank. I find that it helps stabilize the tank and provide extra support if it gets knocked over so the flimsy tank threads don't get messed up or snap off altogether. When I'm using my Viva Nova, I'll unscrew the collar snugly against the bottom of the tank. That's the only tank I've used so far ("Kanger Protank 2" in the mail) so I don't know how much gap there is on other tanks out there. My gap isn't large, but it doesn't rest on the body of my Vamo when it's as tight as I dare go either.
On a different note, I wonder when they'll smarten up and use the wider E-go threads on these tanks instead of the weak, tiny diameter 510 threads most tanks seem to use. They're just so prone to rocking and breakage since the shaft is so thin and flimsy. I hate the way my Viva Nova occasionally shifts, even when it's tightened down snug. It makes me cringe when I think of what is happening to the threads. The 510 threads make sense for smaller applications, but not for a large tank IMO. Most newer PV mods have both style threads, so why not take advantage of the wider/stronger E-go threads instead? They use them for the smaller mini tanks for use on the E-go style batteries, so why not put them on the bigger ones as well? Is there an aspect to fit and form that is causing them to shy away from this that I'm overlooking?