This might be a dead discussion (dated) but I thought I'd throw my 2 cents in:
I own both the Segelei Zmax V3 and the Vamo V3. I bought the Zmax first and had problems with the button, and later bought the Vamo from Fast Tech as a backup until I replaced the button on the Zmax (now fixed). In retrospect, I'd get the Vamo if I had to choose one over the other. Ignoring the button problem I was having with the Zmax, the Vamo has a much easier menu system. It's easy to change wattage or voltage on the fly, whereas the Zmax requires multiple button presses and menu delay while you wait for your option to take. If it wasn't for that, I'd choose the Zmax since it has a stronger and heftier body over the Vamo which feels a little lighter/cheaper in your hand (but definitely not an issue for me). I also have to mention the removable threaded upper collar on the Vamo which is a fantastic feature that I simply love. I use it mainly to tighten the top of the Vamo against my tank to help stabilize and strengthen the joint to reduce the chances of damaging the tank threads if it gets jostled or dropped.