Pressure Pots

Pressure pots are how most artisans reduce bubbles. This is because the force from pressure works in two ways:

1. Bubbles are pushed up and out the sprue channels
2. Remaining bubbles are compressed and/or split into microscopic size so they are no longer visible, regardless of bubble size

We're still bound by physics here though... if there are too many bubbles, they will be pushed into corners and you'll end up with holes or entire sections of your cap missing.

Vacuum chambers

Vacuum chambers are handy, but very limited in use. Vacuum chambers are not a substitute for pressure pots.

Since the vacuum force can only eliminate bubbles by pulling them up to the surface, the effect is limited only to the size of the bubbles. In other words, only the biggest bubbles get pulled up and out.

So why use vacuum chambers at all? This is to reduce the total volume of air in your resin, so there's less likelihood of too much air getting pushed into a corner and ruining a cast.

Why not both?

Some makers use both. Typically the only reason they do is for crystal-clear encapsulations, and this is to minimize the chances of even a single bubble ruining an entire cast.