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It's the unpredictability that is the problem, right? The extra vacuum reservoir volume seems the right way to go to me. Some indication of the vacuum remaining in that volume would also be useful.
Since the pressure difference is quite low (~10.3psi/70kPa) almost any container could be made to work maybe even a re-purposed carbonated drink bottle or bottles. The PVC pipe is likewise low cost, light and inexpensive.
A restriction in the feed to the additional reservoir volume might be necessary to prevent the engine operating with high inlet manifold pressure on start up for an extended period (maybe you'll get a spike in the idle speed or a hard start).
There'll have to be a one way valve between the inlet manifold and extra volume anyway. One source for an in line unit will be the brake booster vacuum check valve found on cars with a turbocharged gas engine.
A pressure gauge would provide a better indication of the vacuum reserve left than a simple switch and light. The GM MAP sensors (there are others which are similar) are really simple and widely available for very little cost used. Feed it 5V and provide a ground connection and the signal can be read off with either a cheap digital multimeter or a panel meter. They are almost linear wrt pressure vs voltage.
You might want to check what pressure the one way valve that traps the vacuum in the brake booster opens at for reference.
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