I'm still using my original V1 LT that I got 10 months ago and that gives a pretty consistent output as long as you don't let the battery get too discharged, which is not a good idea anyway. Of course I prefer my Provari - much better build quality and electronics, but I'm still fond of the old LT (one of my first
mods) and it still does a job.
The Vivi should fit fairly flush on it - I've only got an ancient V1 and the centre connection sticks out a long way on them, so it is not quite flush, but I would expect later versions should be a better fit.