To much cam??
You put in cast "9.5:1" pistons. Did you have the block decked? Me thinks your "9.5:1 383" is closer to 8:1. The 383 block is ALWAYS very tall compared to the blueprint spec so you have to mill the hell out of them to get the piston to where it needs to be. Plus, the 906 heads will have much larger chambers than blue print spec especially with a valve job. Were they CC'd to verify the size and mill the heads to correct that? 383s are harder to "get right". Yours is overcammed only because as you describe it - it's about 2pts less static compression that you wanted.
^^^this
Assuming you used a true flat top and .040" (conservative-sometimes as much as .080") under the deck for TDC, an 0.040"x4.4" gasket, and 88cc chambers (typical for 906 based on testimonies on this website), you're probably closer to 8.4:1 at best. And with 8.5:1 and a 284* cam, that may be retarded (dot to dot isn't degreed) on a 108* LSA = usually low idle vacuum.
These guys are telling you correctly. Timing will *help* ameliorate your idle vacuum, but if you're running ~8:1 compression with that cam, you're likely losing a good bit of cylinder pressure at low engine speeds.
If I read your description correctly, you're using a COMP Nostalgia Plus 284H grind 21-671-4; it even recommends at least 2800 stall, with 9.5:1 compression...this also assumes the cam is degreed to a 104* ICL. On many occasions, timing sets are known to actually retard a cam a couple degrees, and that typically softens up your low end.
If you're dead set against changing the cam, take these guys advice; up the stall and work your timing curve. I'd also recommend checking the actual degree on your cam-make sure it's installed as the cam card advertises.
Oh, and what rear gear are you running?