Is the hairdryer a permanent fix? Anytiem I've tried that, the toy eventually goes back. I guess I never dunked it in cold water though afterwords.

I had to do this (cold water) with a new figure I got.
Used the hair dryer,
I added massive ice to a bowl of water
and whal-la, instant fix.
Hasn't seemed to go back out of place and its been a few days but time will tell.

NOTE: If you do it this way,
remember to place a towel or thin rag (cotton T-shirt works best) over the ice and let
the water seep through before placing figure in bowl or the ice may scratch your paint.

i've tried a few of these things to prevent my undead cumberlain's head from caving in, and after it cools and whatnot. plop. caves.

I find getting vinyl as hot as I can then into cold as quick as possible works best. I also position the figure or piece slightly (about a quarter inch depending on the vinyl) past where I want it, seems to be pretty permanent, have had figures hold that way for months without issues. Also to tighten joints like at the waist I heat the outside then just squeeze and hold it before running under cold water in the same way and it seems to do the job as well.

What I have learned doing this is the following, you can't do any MAJOR fix, sunken or caving vinyl is done and repositioning anything more than a half inch has never worked.

i noticed these major sunken or caved in stuff happens with a lot of clear (mostly clear gid) vinyl i own. i'm guessing that must be a regular occurance...

So the heat-than-ice method, will it distort the vinyl? I have a clear toy I want to try it on but I'm afraid it will leave stress marks or cracks.

I have only done 4 pieces but not a clear. I would assume the vinyl should be the same and react the same, the only difference would be a dye source (I'm not sure what or how) they add for color.
None of my pieces are distorted.