I'm not sure what kind of problem you have and what size ViVi Nova you are trying to connect, so it's hard for me to give you a precise answer. Do you have a
threading problem? Are your ViVi Novas not making proper contact and not firing? Are they genuine Vision ViVi Novas?
ViVi Novas have a 510 connection because they were designed for larger
batteries (510 threaded), but they should work on your eGos just fine. Maybe this picture will help:
I have a slim Vision mini Nova that is the same diameter (15mm) as my eGo. I add a sleeve cone--but that cone is mostly for looks--however, it helps support the weight of the bigger tank and stabilizes it. If you have a slim ViVi--use the smaller cone, if it's the large one, get the bigger one:
If you don't want to deal with the sleeves, you might look into eGo-threaded heads like Kanger T3 or eVod.