I behead my ponies before I boil and boil for about 10 minutes on a general day. I also spritz with some "goo remover", like Goo Gone or Goof Off (which Goof Off is my preference) and scrape the inside with a butter knife that I purchased just for pony making intermittently with the boiling.
I also have a pair of 5in needle-nose pliers that I use for it, but make sure that you have on some nice work gloves to protect from blisters and work-rubs. Life is awful for the next week after when you do more than 4 at one time in one sitting with bare hands and pliers.
If worst comes to worse then I boil for about 20mins to soften the glue entirely- just be aware that some hair may transfer-bleed onto the head, so cut the hair prior to this.
I do not recommend using acetone to help loosen it. I think they use more of a non-toxic epoxy because acetone barely touches it and it makes more of a mess than anything.
I don't like pushing from the outside in because it's so easy to cut or rip the vinyl when the holes are so close together. I have yet to see a pony with nice, uniform holes so I try to avoid it, but sometimes you just have to x.x