Yes, you could "bench sync" the carbs by supplying a common vacuum source to the carb inlets.
I don't see the big difference between that and a standard mechanical "bench sync".
The first approximation toward carb installation is the get the carbs all equal to each other. Mechanical sync does that, as would the suggested method.
The true end goal would be to have the unequal vacuum sources among 4 cylinder intakes to be balanced by slightly uneven slide positions. A common vacuum source would not take these variables into account.
So, if your engine is proven to provide exactly the same vacuum from each intake, the common vacuum at inlet would work as well as a mechanical bench sync.
It occurs to me that (using the proposed shop vac method) the sync ports feeding the manometers would be on the open side of the carbs on the bench, with the vacuum source largely blocked by the slides. Seems to me that a pretty sensitive meter would have to be employed to make measurable differences there. Or, a calibrated restrictor plate would have to be installed and sealed to the engine side of the carbs while sitting on the bench in order to make standard gauges be useful.
As much as I disdain the "drill bit method" for bench syncing, I think I would prefer it over the shop vac method, as it seems less labor intensive than the making the fixtures for the shop vac method.
I'll stick with the light method for bench sync. And, then balance the cylinder inlet runner vacuum on the bike with manometers. It's worked out well so far....
Cheers,
