The plug fixture makes very thin walled cups possible (as long as they don't have any cracks or knots..). When I cut the plug on the machine, which must be tight fitting but not a press fit, I drill a bit size hole in the middle as future x/y zero-reference (also serves as vacuum conduit). You must register inside cut and outside cut very precisely or the wall will be uneven and the cup look cock-eyed.
The other advantage of this method is the part comes off the machine as a finished shape (no tabs of onion skin to cut off) and you can use 90+% of the wood without much loss.
I am not sure if the shopvac will do it, since the holding surface area is already small. For my lamp shades it is only 7 square inches which comes out to 60-70 pounds vertical holding force at 20 inches of vacuum. A Shopvac has much lower vacuum capability which may be O.K. for spoilboard/sheet good duty but probably not enough for such a pod. But a small oil-less vacuum pump is only about $150 on eBay.
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