So all I have to do it use another sim to activate the phone then put in the vz sim?
Activate means different things depending on what you're talking about here. It has been a while since I read up on this stuff, so I may not have things 100% correct. I can tell you that an unlocked AT&T or T-Mobile iPhone 6/6+ will work just fine on Verizon so long as you already have service and a functioning sim.
Activate your iPhone - is what happens when you're first setting up your phone. You just need a working sim in your phone. If your iPhone is locked, the sim has to match the network that the phone is locked to.
Activate a line of service with a carrier - this is when the carrier sets up a new line of service. When you're doing this with Verizon, because of the way their systems are setup, they'll ask for your MEID or IMEI. If it doesn't match something on their list of phones that are registered with them, they won't activate the line.