Here is what I do... Not saying it is the best way...
1) Lift back the vinyl flap just a bit more than needed.
2) Clean the vinyl flap with 50/50 alcohol. Yep...
3) Clean the vehicle area again with 50/50 alcohol.
4) Put a nice layer of 3M Primer 94 down. Let it dry for a minute or two.
5) Relay the vinyl flap and heat it up just a bit after it is back down.
I had a corner on a box truck come up after the install. The box truck had previous repairs to the body and had inserted new rivets. The rivet was extremely close to the edge. This is the area that lifted. After doing the above, it is still going strong in that area months after it was repaired.
I was a bit concerned about the alcohol on the vinyl while cleaning. It will weaken the bond for the adhesive. But it was already in bad shape, so I tried to get all grime off of it. The primer 94 is a great asset for wraps. But a pain in the butt to remove vinyl from heavily coated primer 94 areas. Keep that in mind later...