I was on Verizon in a family plan with my wife. Her's was the main line, mine was added for like $10/month.
I've purchased an iPhone and moved to AT&T. Will I get a $175 ETF from Verizon for the one phone number that I ported?
Yes, I called Verizon about this because I am in the same situation as you:
You will get $175 ETF if you port your VZW phone over to ATT before the contract is up.
It doesn't matter if you keep any other Family plan phone "active" or not, because each phone has it's own "contract" with separate anniversary dates.
(i.e. if you switched 2 phones over from the plan, you would get charged $350)