OK... you've saved and saved and ordered/purchased a new pair of vapours... you've possibly forked out a good £100, yet constantly on your mind is "i hope I dont get blisters". I'm sure you've all heard the "wear 2 pairs of socks!" .... "take a shower in them"...
I have 2 pairs of vapours, (the rosa berry and pine green) both FG's. I am a between a size 9/ size 10, I heard that you're going to get blisters regardless but you eventually wear them in. My friend recomended i buy them a size smaller (nice and tight

they only had them in a 9 anyways, so i bought them, wore them that night, after 40 minutes my heels were killing and I could barely walk let alone run. I kept wearing them... the 3rd time i wore them my feet were fine.
With the rosa's, I got them in a size 9.5 wore them, they fitted perfectly, could get my foot in the boot easy, plenty of room, weren't tight. Wore them for the 1st time and at the final whistle, i realised i wasn't even thinking about if or not i had blisters. comfiest boots iv worn. Didn't affect my feet at all
few weeks on i wore the pine greens (which had been broken in) and got
blisters again

so i would recommend that if you're in the "breaking in" process, you stick to it, don't switch your boots.
My advice, from 1st hand experience is that:
*When you 1st get a pair of vapours, if they are tight, when you're not wearing them, roll up sum balls of paper/newspaper/socks etc and push them into your boots, cover all of the boot. It helps if you get a big sock/ball of paper etc at the back of the boot so it opens the heels up a little bit.
*walk around the house in them, to prevent damaging the blades cover your blades (paper... socks (make sure an old pair as they may get dinted from the blade) and secure with an elastic band/tape.
* (helpful tip!) I always do this even with the rosa berry's... vaseline! take some vaseline with you when you play in your vapours, take some on your fingers and rub around the heel pads inside your boot and the rim of the boot, any areas you fear will hurt your feet. also put some on your actual heels, on your socks. This prevents friction, if your feet start to hurt apply some more, its a big relief!
*wear two pairs of socks, if your boots are tight you may have trouble wearing 2 pairs, i wear some plain white socks over my playing socks and band them at the bottom (simply to keep my shin pads up) with me it didn't make much difference, I would suggest you try this yourself however don't wear 2 pairs if your boots are quite tight, its likely it will just add pressure onto your feet and wont help the blisters anyway. As i say, this didn't make any difference for me but many people recommend it so....
* if your feet DO blister, cover with a plaster/mole skin, make sure you have a plaster on any bad areas when you play and use plenty of vaseline.
To summarise, if you want to avoid blisters use the tips above, These are the ones I've tried and helped me. vaseline is your best bet, they should give you a free tub of vaseline with every vapour purchase! Something you should consider before buying is the size. I've heard many people recommend you buy vapours 2 sizes smaller!!! and apparently Ronaldo is a size 10 but wears his vapours in an 8, whether this is true or not...
But yeh my advice is buy your actual size, maybe half a size bigger, the last thing you want is them being too small. You're just adding pressure and blisters are going to be inevitable, at least if they're too big you can wear an extra pair of socks

I rarely wear my pine vapours (size 9) now as they usually hurt my feet. Yet my Rosa's (size 9.5) have never made my feet uncomfortable or blister, not even the 1st time i wore them. So a .5 of a size goes along way! Which brings to my final point of the actual vapour, I think there's an actual difference in feel depending on what colour of vapour you buy, possibly different material. The pine green, vivid blue don't seem to have that "spongy" feel like the "rosa berry" and the latest "citron"... (the Rosa's and Citrons don't seem to be as shiny, both lace cover and actual boot) weather this is in my head or not I don't know

But I'd maybe try on a few different colour vapours (in the same size) and see how they feel.
Rant over, I hope these tips help and maybe clear up a few issues (and blisters) I haven't tried the showering in your vapours technique... but if all else fails... VASELINE!!!