Yes, that works. The last ability on Lethal Vapors is an activated ability (it's written out as "[cost] : [effect]") and it isn't a mana ability (it doesn't produce any mana and it's not a loyalty ability). Pithing Needle can stop people from activating it.
July 11, 2019 7:50 p.m.
I admit again I pretty much already knew, but it felt "fun" enough to share.