To Vacuum Advance or Not to Vacuum Advance

Here's another thought;
Say you had timing all in by 2800, and V-can hooked to full-time vacuum. What happens when you slam the throttle shut at some higher than all-in rpm, and the car now drives the engine with it's closed throttle,(compression braking) and the timing is say55 or more degrees?
Meh, maybe nothing.
I didn't want to find out, so I'm on the spark-port.