I noticed on my recent RTW trip that no passenger gasper vents were installed on any SQ aircraft, nor on QF 767s (ex-BA ones anyway), nor on ANZ 747 or 777s.
A benefit of not having them is that the cabin is definitely quieter due to less hissing air, the downside is that the cooling in the cabin ends up being more "one size fits all".
They definitely seem to be falling from favour on many airlines, as beside making for a quieter cabin, they also are one less thing to maintain.
If you are hot and uncomfortable, tell a member of cabin crew. It's much easier to cool the cabin down and for the colder people to have an extra blanket, snuggle up under their duvet, or put on a sweater. What's the other option? Stripping to your underwear?
Having said that, I was on a VS flight where a girl in the next suite slept in just vest and knickers, but kept herself covered up with the duvet... no-one said anything.
Cheers,
Mike