I ran into the same problem with a vial of burton ale a couple of months ago. It took persistence to finally get it loose. I did the "ridiculous amount of shaking" thing. I also had to smack it against my hand a few times, roll it around and shake from every conceivable angle. It took about 5 min to finally get the yeast free from the sides of the vial.
It must be a good flocculator. After all of that, it left a solid, compact yeast cake in my English IPA.
As long as you haven't gone well past the "Best By" date, I say just keep shaking until you get the majority of the yeast free.
BEWARE - Make sure you vent the vial by opening slowly, a bit at a time to release the CO2 after you're done shaking. Unless a yeast facial is what you're going for.