I have a ProVari and have used an eGo type batter in the form of Johnson Creek's VEA system ... I mean there's no comparison honestly between those two, but if you've got the funds, get the ProVari, you won't regret it. I wish I had found ECF and done my research long before buying the VEA, (also tried a couple other companies "look and feel like cigarettes") I would have saved restocking fees, etc etc.
As for tanks, I have used the Vivi Novas as well, up until I got this new carto tank in from
IBTanked.com I love it, got the XL which holds about 2.5/3.0ml of juice and reasonably priced too. ($25 bucks for the clear tank and Stainless Steel caps) They offer a standard size tank as well, but not sure how much juice it actually holds. Customer service is terrific and it appears to have just changed their website (from the time I ordered last week) with a few more selections.
Don't get me wrong, I still use the Vivi Novas, their latest tank is quite good and with the ProVari using the 2.8ohm or 3.2ohm heads, you can't go wrong on the 3.5ml tank. Not to mention it looks like it was made for the ProVari. On a typical day, standard weekday, the 3.5ml tank will last me all day. On the weekends, when I have full on freedom from work, etc ... I tend to be more of a vape-a-holic.

and may have to refill ... but typically, I'm good to go for an entire day.
I guess it's all in what you like and what best enhances your vaping experience. Good luck with whatever you choose, I know there's a ton of good stuff out there and what I wouldn't give to win the lottery to try each and every product that someone has given thumbs up to. <heh> Keep us posted on what you decide.
EDIT: oops, I see I took too long to respond, and you already have 4 of the standard Vivi's ... DOH!