Still think that information belonged in the stat block or at least the dossier though.
I agree. Of course, I also think that if "Subtle Spell" was meant to work against counterspell, it should have been more clear. Personally, I don't think that "No VSM" means "No one can tell that you're casting a spell" (Certainly not VECNA of all people).
I mean, let's take most visual representations of "psychic powers" that we have in many media. Even if ALL you're doing is "willing" reality to break (IE Casting A Spell), you usually
still show some effort on your face. Psylocke's face has a purple butterfly! There's virtually LIMITLESS fluff for it. Sure, no VSM would make it fairly easy for you to hide that you're casting. Fools that know very little about magic probably wouldn't see it. But VECNA? (Or any significant spellcaster? - Personally I think anyone who's good enough to know counterspell could probably do it.)
A DM could easily have a case to rule any of these:
1) Subtle spell is so subtle that no one (at all ever) can tell you're casting (very generous to SS)
2) Subtle spell gets rid of VSM so most people can't tell. To know, they need to roll Insight vs your passive Deception or Spellcasting dc
3) You need to hide it or they know. You need to roll Deception vs their Insight
4) It just gets rid of VSM, so you can cast while tied to a chair and gagged, but everyone can tell you're casting a spell. (Cruel to Subtle Spell)
Personally I think 1 is as unreasonable as 4. Certainly Vecna (of all people) can probably tell. Again, I think manipulating reality/tugging on the weave/whatever should take more effort than just wandering around while stuff happens without anyone knowing where it came from/who did that. Even if you're subtle about it.
(Though I would definitely let a player with Subtle Spell get away with it 9/10 times. Only with particularly awesome spellcasters would I probably make them roll-off.)