Answer from FriendOfMrGolem120 on Smogon:
Haze is rarely used in GSC. The most "common" examples I can think of right now are Qwilfish in UU and the defensive Dragonite set in OU (Reflect, Haze, Sleep Talk, Rest; which is really rarely seen and very vulnerable to critical hits at a bad time). These two actually illustrate the different uses of Haze perfectly: on Qwilfish it is usually used as an emergency check to a set up mon. You use it when such a mon becomes too damgerous and after Qwilfish eventually faints, you can bring in another Pokemon safely.
Defensive Dragonite on the other hand tries to be a longterm counter to a few Pokemon. In order to do this, it is important that it doesn't take too much damage from non-boosted attacks of the Pokemon you are trying to wall and that you have a recovery move like Rest.
- I have never seen someone use Haze an a boosting move on the same set I think.
- This is actually an important question that I already tried to adress above a bit. Yes, your opponent can stay in and just use the setup move again but then you could also just Haze again. If Haze has more pp than the opponent's boosting move, boosting won't help your opponent. You were speaking about Vaporeon as a Hazer and Snorlax as a Curser. The issue in this matchup is that Snorlax already threatens Vaporeon without boosting so it is no long term solution, just an emergency one. This is true for the Dragonite matchup as well though so I see a flaw in my argument but on the other hand, I doubt you were planning to use a Vaporeon set without any damaging move and the same set as D-Nite. And to be honest, Vaporeon would do that role quite poorly. In OU the most common Vaporeon set is Surf, Growth, Sleep Talk and Rest. Acid Armor is also used instead of Sleep Talk. There are some other good sets as well, but none of them include Haze.
- Depends. If you have multiple Pokemon that could handle the opposing boosting mon, you can switch between them. If not, you usually stay in. But once again, in practice Haze doesn't play a large role in GSC OU.
I don't recommend to use Haze on Vaporeon.
PS. There is also Kindgra as "viable" Hazer in GSC but well... it is not really that viable.