[alexiskai]

It's possible you could take the car to a sympathetic auto repair place, since vacuum leaks are still a thing that afflicts modern cars and they should have equipment to diagnose them. Alternatively, applying flammable or non-flammable sprays carefully with a straw may help pinpoint the leak.

I like the idea of a crack that might only be opening up when the manifold is bolted to the block, maybe something very close to the bolt site.