Well, this is my theory on why you might experience nasty blisters on the heels of two boots: Nike AZT 90 III and Nike Vapor I/II. First, I'd like to start off by saying that it's normal to experience a blister or two when breaking any boots in, however, I noticed as I was cleaning the AZT 90 IIIs that the heel was creased near the achilles (ex. Vapor I/II). You can see and feel the material expand near the crease (~heel) and this causes uncessary friction. I've been playing in pulses for a couple months and last week I used my AZT 90 IIIs in a pick-up game and it gave me some nasty blisters, one the size of a quarter on my right heel (right footed) and one the size of a dime on my left. This happened to me with the vapors as well. It wasn't as bad with the vapors as my heel had callous'. (The heel on 95% boots are constructed with a U back) I'd like to point out I didn't use these boots since January and my heels probably softened up, but nonetheless it has been a painful experience.
It probably is because your heel has softened. You have to remember that vapors and/AZT90s have different shaped backs to the pulses. So where you built the skin up would now have got tough around a different part of your foot. I' got a nasty blister the first time i wore a pair of nike ID AZT90 right where the heel cup is. Although it hurt i dressed the plaster when i played and after a few games it was fine. If the blisters are really painful i'd let them settle down before you try and wear them again though. Try soaking your feet in warm salt water this will toughen up your feet the more you do it. I got NASTY blisters off some copas. I think i must be the only person in the world to get blisters from them. But it took the skin (thick skin) off the balls of my feet and i ended up having to go to hospital and having both of them dressed. If they hurt the next few times your wear them, then maybe theses round shaped boots aren't for you? All I can say if you like the boots keep trying but if they make your feet so bad. I think you need to look into getting a different boot. You can always tape over the 'crease' in the boot. Use some general band aid tape. And see if that does anything to help? appleCORR