Well that's one more lesson learned (thousands more to come!) about the frustrations of this game! I don't bother trying for more than a minute or so with the dry vac - doing it for longer is not going to change the fact it won't come off with a vac! Mechanical agitation after pre-spraying is probably the best - the carpet is lubricated so you won't damage it so easily. A rotary with scrub brush probably the most effective, or maybe a decent CRB machine (a Duo won't really get in to it well enough)
Normally you can get a pretty good job done with this on a polyprop because you can scrub the crap out of it (relatively) compared to nylon and wool, which are a bit more difficult - the amount of agitation needed will wreck the pile if you're not careful.