Hi Tootster and welcome to the forum.
I have both devices, but in slightly different versions.
For me, if the Zmax is a Sigelei (as you indicated in your post) then I would probably go with that over the Vamo. The simply reason is the Zmax is slightly better in overall quality.
The Vamo is a good device and the features and functions of the two are very similar, as are the forms of them and their ability to provide a good
vaping experience. The Sigelei edges it in terms of quality of threading, thickness of the tubes, finish, and better quality connections.
Only thing to consider is the Zmax has a one button interface for changing settings and the Vamo is a 3 button setup, making it slightly easier to navigate the menu. Once you get used to either device, this really isn't that much of an issue.
Just wanted to add, where are you going to purchase them from and how much are you expecting to pay? You might also want to consider that there are other devices out there now that probably offer more features for similar money.