No, the metering of fuel is done by the float, the prime just keeps a constant flow to it. The Vac will stop all fuel movement when engine isn't turning over.
Downside to prime is if a float sticks (which they do) then the freeflow will just keep going & dump all that fuel thru carbs into & past the cylinder/pistons into your oil
With Vac, the fuel will freeflow if the float sticks, but as it's running, it will mostly burn it off before getting past the pistons, & will stop flow when engine is shut off.