Why is my manifold vacuum so low?

Bad porting can really hurt an engine. But so can an MP produced cam that wasn't degreed. I think your issue is less of the static ratio, and more of the tossed together and not blueprinted. Tuning will also be a big deal. the timing curve it's going to want will need to be set into any distributor you might be running. If you just dropped something in or used stock then that's hurting you too. Lastly is the carb tuning. Once the timing is set, then properly tune the carb. I'd expect a smooth idle around 750rpm, with at least 12-14" of vacuum if not more. Unless the porting sucked...

Edit - the booster was bad- makes sense. But vacuum is also an indicator of just how good the engine is really running. IMO, you're low, so potentially missing out on some power and economy (and emissions given your neighborhood). But you're the customer so if you're happy...lol.