I don't think there really is. The benefit of Flyby Attack seems to be the reduced prerequisite burden. Anyway, since these should already have Flyby, let's give them Imp Flyby rather than Spring Attack, just for the novelty of it.S
Looks ok!
You have any ideas for flavor text? Tactics would presumably be to void storm and slam while flying by.
Oh, don't forget the slam attack bonus.
Air-based spells (such as gust of wind) cast by a wizard or priest of a level higher than the quasielemental’s total Hit Dice slay the creature if it fails a saving throw versus death magic.
...
Air-using spells can destroy a Vacuum quasi-elemental only if the level of the caster (or hit dice of the monster) is greater than that of the Vacuum quasi-elemental. If attacked in such a fashion, the Vacuum quasi-elemental is allowed a saving throw vs. death magic; if this succeeds, it is unharmed by the attack. If the saving throw fails, the Vacuum quasi-elemental is destroyed. A gust of wind cast by a 1st-level magic-user has no effect on a 6-HD Vacuum quasi-elemental, but a 7-HD djinn could destroy it.
I did in fact mean putting in the numbers.
Do we want to do anything with this?
I think I'd use a CL check or straight spell level rather than caster level, but what do you think?
Hmmm. This is getting a little fiddly. I think I'd prefer just caster level (or just spell level). Then you only have to worry about casters of level equal to the quasielemental.
Let's go with the ability you wrote but using caster level.
Tactics: Vacuum quasi-elementals are usually quite hostile to strangers, flying past and attacking with slams. The presence of an atmosphere seems to enrage them, and they typically use the full duration of their void storm as soon as they are exposed to sufficient air.