to be honest, if u wanted a mod that is the size of the ego but with VV, u can do the ego with the preset voltage changer or use a ego booster. either way the battery life is too short. it's a bit much to be asking for something ego sized with legit VV. to make an option to extend battery life from the 4 hour vape to 8+ would probably end up making the envy one of the longest PV on the market. like i said, you either have to thin it out or shorten the mod. thinning it out means lengthening the battery, shortening it means making it thicker. The smallest possible form factor for a VV tube mod is probably going to be the thickness of the Ipro unless they develop batteries thinner than 14xxx's with a higher mAh rating than the 14xxx batts currently.
In all honestly, you should be using batteries with at least 600 mAh with VV. consider how many times you'll have to charge the batteries in a day with only a 4 hour MAX vape time. in the long run, you'll end up buying batteries frequently and will in turn make u spend more money. Even if you buy more batteries to cycle through, that would mean you would have to keep opening your bottom cap to change batts as opposed to just buying more ego/510 batts and just switching them out From what i'm thinking, the 4 hour max vape time will only happen if your drawing a very low amount of amps for vaping (1 to 1.5 amp). If you are going to vape at the point of drawing low amps, you should just stick with the egos because the purpose of VV is to be able to use whatever u want on it at whatever voltage and if you get a MAX vape time of 4 hours on low amps, you're going to have a big drop in vape time vaping on other settings that draw more amps.
As of now, i think the most reasonable thickness for a VV tube mod is probably the Ipro because the 14xxx batts are the smallest batts to use with VV that also has the ability to give a decent amount of vape time even with setups that draw 2+ amps.