So how does the vacuum leak down from 25 to 15"? There's a small leak somewhere, but probably not bad enough to cause major issues.

You may have a check valve failed somewhere or a vacuum connector rotted out that leaks when under high vacuum.

The critical troubleshooting element....how's your vacuum with everything hooked up and the engine running? Lousy vacuum from the vacuum pump or a leaking brake booster will cause system-wide vacuum issues.