From the Book of "Its Never The Carburetor", check everything else but the carburetor first.
My first bet is that the gasket doesn't have a hole where it belongs, given that the vacuum reading is low and the vacuum advance doesn't do anything.
My second bet is that the coil is heating up at the 20 minute mark and is crapping out. Once the coil cools off, a marginal coil will make a spark again.Check for spark at a spark plug as soon as it dies.
