Good point. Shall we just take the air elemental feat progression? It's not too different than fire's anyway. Other than Flyby Attack (which the vacuum should have), I think the only difference is what order a few feats come in.
We can't use the Air Elemental as-is, because the Quasi-elementals have different Hit Dice than regular elementals. It only makes a difference to their number of feats at the Huge and Greater sizes though.
Also, the SRD Air Elemental doesn't have Dodge after size Huge, making its Mobility and Spring Attack feats illegal because they don't have the feat prerequisite. We'd better not do the same with the Vacuum!
Air Elementals have the following (with the additional feats at each stage
in blue):
Small: Flyby Attack, Improved Initiativeᴮ, Weapon Finesseᴮ
Medium: Dodge, Flyby Attack, Improved Initiativeᴮ, Weapon Finesseᴮ
Large: Combat Reflexes, Dodge, Flyby Attack, Improved Initiativeᴮ, Weapon Finesseᴮ
Huge: Alertness, Combat Reflexes, Dodge, Flyby Attack, Improved Initiativeᴮ, Weapon Finesseᴮ, Mobility, Spring Attack, Weapon Finesseᴮ
Greater: Alertness, Blind-Fight, Combat Reflexes, Flyby Attack, Improved Initiativeᴮ, Iron Will, Mobility, Power Attack, Spring Attack, Weapon Finesseᴮ
Elder: Alertness, Cleave, Combat Reflexes, Dodge, Flyby Attack, Improved Initiativeᴮ, Iron Will, Mobility, Power Attack, Spring Attack, Weapon Finesseᴮ
Monolith: Alertness, Blind-Fight, Cleave, Combat Reflexes, Dodge, Flyby Attack, Great Cleave, Improved Critical (slam), Improved Initiative, Improved Natural Attack, Iron Will, Mobility, Power Attack, Spring Attack
I suggest we fix the feat prerequisite problem by swapping the Elder's Blind-Fight for Dodge.
Then all we need do is cut one feat from the Huge and Greater stages. Spring Attack is the obvious choice, since Alertness and Iron Will always crop up at those stages and Power Attack is just too useful. That'd make it:
Small: Flyby Attack, Improved Initiativeᴮ, Weapon Finesseᴮ
Medium: Dodge, Flyby Attack, Improved Initiativeᴮ, Weapon Finesseᴮ
Large: Combat Reflexes, Dodge, Flyby Attack, Improved Initiativeᴮ, Weapon Finesseᴮ
Huge: Alertness, Combat Reflexes, Dodge, Flyby Attack, Improved Initiativeᴮ, Weapon Finesseᴮ, Mobility, Weapon Finesseᴮ
Greater: Alertness, Combat Reflexes, Dodge, Flyby Attack, Improved Initiativeᴮ, Iron Will, Mobility, Power Attack, Weapon Finesseᴮ
Elder: Alertness, Blind-Fight, Cleave, Combat Reflexes, Flyby Attack, Improved Initiativeᴮ, Iron Will, Mobility, Power Attack, Spring Attack, Weapon Finesseᴮ
Monolith: Alertness, Blind-Fight, Cleave, Combat Reflexes, Dodge, Flyby Attack, Great Cleave, Improved Critical (slam), Improved Initiative, Improved Natural Attack, Iron Will, Mobility, Power Attack, Spring Attack
Alternatively, we could give it Dodge or Flyby Attack as a bonus feat, in which case we'd have to give it an additional feat at each size category.
Hmm, that'd allow us to give it Improved Flyby Attack somewhere around Large or Huge. It's tempting, although it would make it a better "combat flyer" than an Air Elemental, which doesn't feel quite right.
I'll mull it over a bit.