this low vacuum is very common with the vanagon engines but I don't see it tied into a combustion gas leak into the coolant. If the engine was overheated and the inner gaskets received to much crush caused by the cast iron cylinder expansion then the that would be one explanation. The other possibility is a cracked cylinder head. you stated that the engine is only 10K old did the rebuilder use OEM heads they commonly crack and recrack when welded and are known to have uneven head gasket surfaces. I had a 85 that would seal the inner head gaskets once the engine would warm up and you could drive all day without problem but I did need to burp the system after the engine got up to temp. This can be dangerous I used a small valve tied into one of the cloth hoses to do this and dont recomend burping the coolant system with the pressure cap
 
The compression could be 140 150 cold and 80 hot if you are reading the vacuum hot and the compression cold then I would consider adjusting the intake valves to .006ths cold and testing the vacuum again hot. 

Bob Donalds
Boston Engine