Putting system under vacuum doesn't actually suck air out. In reality you lowering pressure to change boiling point to pull vapor out. Just like pressure cap on cooling system raises boiling point. Anti-freeze doesn't raise boiling point pressure cap does
True, however the end result is that air previously in the system is gone after a vacuum bleed.
You can bleed the brakes/clutch via the reservoir the same way you bleed a cooling system after a flush. When you apply a vacuum to the reservoir, trapped air gets sucked out the system(albeit slowly) and is actually the most thorough method for removing air. Because there are no collapsible rubber hoses and because the system is sealed on both ends, there is no flow of fluid during the process. What happens when a vacuum is applied is the air gets dissolved in the fluid then evaporates at the top of the reservoir. The same principle applies to the A/C system during a evac/recharge except the Freon is a gas at normal room temperature and pressure.