Just a thought from the old days of troubleshooting...does the vacuum drop further as you raise engine speed from idle up to that point where the engine just goes off the rails? If so, then I'd take a hard look at the exhaust system to see if something internal has failed and is plugging the exhaust.

Way back in high school my crusty old auto shop teacher told us: If the vacuum drops as the engine speed rises there are three reasons...plugged exhaust, plugged exhaust and plugged exhaust. Over the years I learned that he was right...at least about this one particular thing.