[Christ]

Any line that is coming from the intake manifold will do, as long as it's not valved or restricted in any way.

If you need a line specifically for the vacuum gauge, get a threaded barb, remove your throttle body, stick a vacuum nozzle in there, drill/thread a hole for the barb, while the vacuum is on, so you don't get any metal shavings in the engine. Screw the barb in, then plug the line onto it.