looking at the V3 and the V5.. I have both. There are some very slight differences. The vamo V3 I have is gunmetal, so I wanted to get the Stainless Steel, so the difference between the gunmetal Vs. the stainless is what I can review... first, cosmetically, the V5 looks better. The V3 has better and much smoother threading than the V5.. there was even some metal shavings in the threads when I got the V5, and the threading is sharp.. The 510 connector sits higher than the top cap, so when I put my AGA-T2 on it, there is a noticable gap. I usually unscrewed the top cap on the V3 to mount flush tot he atomizer, but on the V5, I have to unscrew it even further leaving a 1/16th groove. The buttons seem a bit looser than the V3. Another noticable difference, is on my V3, there were two extention tubes, the V5 has the top part stuck to the head unit, and the other extention is for the 18650. Not a major thing, but I do clean the battery connections from time to time, and the V3 is easier. The V5 also has both RMS and mean data, whereas my V3 didn't have that option, however I hear it could just be my firmware version. Overall, I like the looks of the V5 so much better than a gunmetal colored screw looking version. I use the V3 at home, and the V5 anytime I go anywhere.
I ordered the V5 from Fasttech using the Chinapost/USPS option. order was placed on dec 21 2013 and arrived January 9'th.. which is pretty good considering it's from China. I ordered the V3 way back in July 15 2013, and it arrived August 19, about a month, but they also had a holiday during that time, so Delays were expected. It takes a bit of patience with FASTTECH, but I think they're overall a good company.
If you order it, do not order a battery to go with it. Do that in a separate shipment and order the batteries separately with something you wouldn't mind being delayed. Also, if you do order batteries with fasttech, make sure you get at least two of them and make sure you order it with something else so it doesn't look suspicious when it goes through an X-ray. They wrapped two batteries of mine in tin foil, and they passed. Can't guarantee that now.
good luck, and vape on.