Just a safety note here: do NOT use any sort of glass jar, especially some random $10 thing for a vacuum flask. If they break, they implode. Almost instantly, with staggering power. (15 pounds per square inch of surface. Do the math.)
I once had a real glass vacuum jar, from the bad old days when they still made such things. About the size of a 2 liter soda bottle. It finally cracked on me one day. Blew chunks through the drywall. It ‘squeaked’ a little just before it blew, which gave me just enough time to hit the deck.
The bigger they are, the bigger the bang when they break. I have a friend with one from the 1890’s, that’s about the size of R2D2. He won’t pump it down. He’s calculated that if it were to blow, it would have roughly the power of a stick of dynamite.
There’s a reason that nobody still makes vacuum jars out of glass, and hasn’t since the high strength plastics came along. For the $50ish they cost, just go get a real bell jar. You’ll live longer.
As for the rest, opinions differ. I’ve built a number of vac casting rigs, and worked with spin casters for years. For one-off, I prefer spin casters. You get better metal density, and you have more force to slam the metal into fine detail.
For large production batches, vacuum is the way to go. Much safer than trying to sling a few pounds of molten bronze around in a spin caster. (I once worked with one that could handle several pounds in a shot. Which was great…unless you had a blowout. At which time it got downright festive.)
The funny ‘rubber’ pads are actually silicon, and available from any of the tool houses.
If you do make one, remember to put a trap in the input side of the vacuum line. (dig through the archives looking for my username crossed with ‘vacuum’. I did a pretty solid rundown on homebuilt vac casting rigs at one point. I remember spelling out the details of the metal trap and the pump requirements.)