"Some of you still do not understand this."
Translation:Někteří z vás to stále nechápou.
But we have no "vy" here, we have "někteří" as the subject.
Notice that in English you also say "someone/everyone understands" and not "someone/everyone understand". That is not exactly like the "some" here, but you get the verb agreement too, just a different one.
Some(one) understands - některý nechápe. (one person)
Some (of some group) understand - někteří nechápou. (multiple persons)
First note that it is not used as a demonstrative in front of a noun here. "If it were I do not understand this performance." I would use "Já nechápu toto/tohle představení.".
TO is a bit special, or at least different from ten and ta, as it also appears in the "to je" / "je to" constructions where it means either this or that.
I would consider this an exception where we would choose TO when speaking something that corresponds to English THIS. You can, of course, use tohle or even toto, but to is much more likely. Check https://syd.korpus.cz/F7cz43hV.syn