The Helium tank would be overkill- - - -those things are designed to hold 2K PSI or so and they're very heavy. PVC pipe is readily available and cheap, and the end caps are domed- - - -the perfect setup for resisting the considerable force atmospheric pressure exerts on a vacuum tank. A 20" vacuum means that every square inch of surface area of a vacuum reservoir has about 10 pounds of crushing force exerted on it- - - -several hundred pounds, total. Connect manifold vacuum to an empty square-sided 1 gallon paint thinner can sometime and watch what happens- - - -the can will be crushed into an unrecognizable hunk of wrinkly metal.

By putting a rubber grommet into one PVC end cap for a brake booster check valve and drilling and tapping the other end cap for a hose nipple, a very efficient vacuum reservoir can be fabricated. Paint it black and it just about disappears into the C channel of a frame rail.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!