It occurs to me that you don't even need Fi. Just keep your Verizon number and gv or hangouts to receive your gv calls and you'll be fine.
To get the benefits of Project Fi, (automatic switching between tmobile and sprint), you need a a phone number on a project Fi sim, in a project Fi phone. To get a number on your Fi sim, you can port over your Verizon number, port over your Google voice number, or get a new number.
I ported over my gv number to Fi. But every device I own: three iPads, two computers, an old iPhone, and two old Android phones, one with an at&t number and sim, are ALL set up to ring when someone calls my Google voice number, even though my Google voice number is now also my Fi number. So I just answer whichever one is closest to me when I hear it ring. Just install hangouts on anything you want to ring with your gv or gv-fi number.
Does that answer your question?
-Peter