A blocked breather on the fuel cap wouldn't cause a vacuum after just a 30 second run but I think you are right, it is probably fuel related.
I would check the fuel pump, a weak or porous diaphragm could cause a lack of fuel. I would also try and put an airline on the fuel pipe and blow air back into the tank to make sure there isn't a blockage.
You could fit a small temporary fuel tank under the bonnet and run the car just using the throttle linkage to see if it drops power, if it doesn't I would fit a new fuel pump, they are not very expensive.
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