You're never going to see a small vacuum leak with a cheap dampened gauge.
You MIGHT see the effect of a small vacuum leak using electronic vacuum sensors. Maybe. The one on my 'scope can match intake-valve-open vacuum pulses to the correct cylinder creating it; but I've never used that to confirm an intake gasket leak.
Years ago, they were selling ultrasonic microphones for detecting vacuum leaks and window-seal air leaks. But you'd likely have to cram the microphone inside the intake runner, or work it in from under the manifold. Totally impractical.
A smoke-test can often confirm very small leaks. If the gasket is leaking on the bottom-side, you'll have to blow enough smoke in there to fill the crankcase before the smoke will come out the oil-fill cap opening after you remove the "710" cap.
If you don't know how old the intake gaskets are...time to pull the intake manifold. The Vortec engines had tremendous problems with intake gaskets when they were new. I thought the gasket was "improved" to solve that issue; but I'm hearing on this forum that it was never "really" dealt with--better, but still not "fixed".
I've lost track--have we completely ruled-out injector and ignition problems? Valve springs, cam lobes good?