Correct, a high source location will NOT work for a distributor VA nor will a correctly located source with a bleed-off slot next to it.
Most Q-jets made in the EGR years will have one or the other. The ones using a bleed off slot are easy to convert to full ported vacuum for the distributor by simply filling the slot with Marine Tex or the entire area behind it but keep the slot open to engine vacuum and to the lower left vacuum fitting on the baseplate.
When I custom tune here in a lot of scenarios I prefer a CORRECTLY located ported vacuum source to the VA vs full manifold vacuum.
There is a LOT of miss-information out there on the subject of VA, like ported was only done for emission reasons, and that MVA makes you engine run cooler, or you absolutely MUST use MVA or you are an idiot, etc, etc.
What I've found here is that following much of the miss-information on the NET folks try to use MVA and run the timing clear off the scale as a "crutch" fix for WAY too much camshaft for their application, LSA way too right, and more times than not insufficient idle fuel to the mixture screws at the relative "low" vacuum their engine build is producing.
IF you find your mixture screws unresponsive or very little control and you need to advance the timing clear off the scale to make your engine happy at idle speed simply "tip-in" the choke flap or gently place your hand over the front of the carb. IF the engine speed increases idle is smooth you need to fix that issue BEFOE trying to run 20, 30 or even more timing at idle to correct your issues.
Once the carb is up to par you will find that the engine may not want, like, need or respond well to a butt-load of timing at idle.
Of course none of this is going to apply to engine where very poor choices have been made for compression and cam selection. For most well though out engine builds you will find initial timing around 8-14 degrees will work just fine and provide a smooth steady idle in and out of gear with very little RPM drop, and easy cranking on hot restarts........
