you use your vacuum to do two different operations.
To degass the liquid investment, this has to be done in a limited
time and down to a vacuum that will boil water at 20deg C. The size
of pump is a relationship with the size of tank. Ie bigger tank,
bigger pump. Or more accurately in relation to the amount of spare
space in your tank when the flasks are in it. The speed of evacuation
will drop as the oil gets dirty or as miniscule leaks develop.
To cast, you don't need a hard vacuum to cast ( in fact I just
did an excellent cast with a sudden leak when a seal slipped with a
vacuum of probably only 1/2 qn atmosphere(0.5 Bar).
But you are always going to do both so you need a powerful high
I have a couple of systems, the smaller one has a cyindrical tank 8"
in daimeter and 14" deep, I use a 56 litre per minute pump which just
about copes. You can always play about with bits of plastic pipe,
tubes are very strong and you cannot exceed one atmosphere, plumbing
fittings are cheap and can easily be sealed with silicone caulking,
just make sure that your pespex top is thick enough, for an 8"
cylinder 1/2" to 5/8" is sufficient.
I have a big reservoir for larger casting that is a piece of plastic
drainpipe about 14" in diameter, with an inch thick perspex lid, I
never ever bothered to glue the bottom on, a piece of sheet
stainlesss steel and a rubber seal and the vacuum holds it together.
The old fashioned rotary oil filled pumps are a great workhorse, you
need to put a lot of oil in them but they are very forgiving of water
and contamination( plaster dust). The more modern rotary vane ones
don't like water in the oil and need to be run for a long time after
you have degassed otherwise they wreck the bearings (expensive!!)