Aircooled VW South Africa

It is highly unlikely that any pressure will build up in the "vacuum" chamber; it is not a hydraulic lifter. The piston is a rigid press fit in the follower and if it were to move in any way it would it would damage the follower and you would have a very noisy valve train. If you were to drill the follower as indicated you would cause a further problem in reducing oil flow to the heads as there would be less oil flowing down the push rods, and loose oil pressure to the rest of the motor. You need to remember VW engineers spent many man hours perfecting the follower, infact the original German factory lifter is far superior in quality to 95% of aftermarket followers out there today.
The yanks have a Scat lifter that works using something similar to what you are thinking called Lube-a-lobe lifter it as a 0.6mm hole on the face of the follower and the idea to lube the cam lobes during operation, but this follower is specially designed for this purpose it's not just a drilled stock lifter.
Have you opened the motor yet, do you know what is in there? That will go a long way in resolving the problem.

I have seen type four cams not make it past the engine stand before the cam was junk. Those little pistons are such a press fit that it will not move from normal oil pressure. Set the clearances cold and check this first
I have seen type fours with valve seats coming loose and valves stretching after very few kilos. Show us your cooling set up