Either plastic air brake lines, or the much less expensive 1/2" CPVC plumbing pipe will work for vacuum lines that will never rust or corrode. Just be sure to bracket it securely to the frame rails so it doesn't sag or get vibration cracks. The cream-colored CPVC resists freeze cracks much better than the plain old white PVC if the lines happen to accumulate some moisture. I use it routinely as conduit for trailer light wiring.
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!