The VK carb does have the port, it's just blanked off from the factory.
I recently did the same conversion and was lucky enough to have some help from an old skool mechanic that works at the workshop that reconditions and re-graphs distributors.
I took my reco'd Varajet to him and he drilled the port out and placed a hose connection in the now un-blocked port.
It had been a while since he had done one, so I watched him compare the VK carb to a disassembled VH one. The hole for the vac port on the outside of the carb isn't blocked off, it's blanked inside the carb, so don't drill into the side of the carb hoping you'll jag it. The internal gallery turns 90 degrees upward about 1/2 inch into the body and that's where it is blanked off.
As far as taking the vac feed from the manifold, here's my take on it -
At idle, your in dash vac gauge will read 6~7 (10kPa).
At WOT, it will read 1 (10kPa).
As you open the throttle butterfly, the manifold breathes through it, so the vac pressure in the manifold drops.
This will mean at idle you have MAX vac advance on your timing and virtually NONE at full throttle. Arse-about-face.
The vac port on your carb is ABOVE the throttle butterfly. So -
At idle, it will have 0 to very little, vac pressure. ( It isn't being effected by the manifold pressure. The butterfly is closed, like a door, all the action is below it).
As you increase throttle, the butterfly/door opens, exposing the ported vac to more manifold pressure until WOT, when you will have full pressure, hence FULL vac advance.
As I said earlier, this is the way it makes sense to me. I know plenty of guys just hook up to the manifold and away they go.
Bosch no longer manufacture the Vac Advance units and the diaphragms perish. If you have one in good condition, look after it!
Hope this gets the brain working. Cheers!