It doesn't pool, it is actively displaced. You use a relatively close-fitting "pusher" block on top of the aggregate, and small holes in the ends of the form itself. You essentially enclose the whole thing in a bag. When you draw vacuum on the bag, unlike in a rigid chamber you get two things to happen (other than not needing a giant rigid chamber!) - first the aggregate is compacted evenly at almost 1 bar, or almost a ton a square foot, by the pressure differential. Secondly, this squeezes out all of the excess epoxy rather the same as if the block was being pushed down by a hydraulic press, which can also be used for additional longer-term clamping if needed so you can turn the pump off and not lose holding force.
The excess epoxy needs to go somewhere, so to keep it from getting into your vacuum pump you use a "drool can", essentially a short chunk of PVC pipe with two end caps and hose fittings. The vacuum pump hose attaches to a top-mounted hose barb, the hoses from the form to side-mounted barbs.
This allows any excess liquid to pool in the bottom of the can while vacuum draws out through the top, this saving your pump and all but the short chunks of hose to the form.
This is how even thin, curvy items get made as well as thick ones - the bag then just goes around two blocks with mating shapes with the epoxy filled matrix in between. The vacuum "clamps" the two blocks together and squeezes out excess epoxy.
This makes for thinner, lighter, stronger, more consistent thickness race car and aircraft panels than old-fashioned hand layment.