I’ve never seen valve covers with addled “insulation†on the inside. Based on that alone, I’m suspicious of the so-called rebuild.
Re vacuum: you can’t be sure if your 15†reading is good or bad without knowing your cam specs. I have a 10-11†with a quivering needle...and power brakes. Obviously, my can ain’t exactly stock.
Others more knowledgeable will off more detailed advice, but you need to pull the heads for a fresh valve job, and while you’re in there, might as well yank that cam. You know you want too...
I'm a hobbyist. Not a professional. Don't be hatin'!