not sure which type of vacuum is needed, but the closer to the anode the cathode is, i'd imagine the less vacuum required.
any easier alternatives to a vircator? i cant think of any at the moment. some were experimenting with terahertz output using vircators.
it would seem if the vacuum weren't an issue, they should be easy enough to make.https://en.wikipedia.org/wiki/Explosively_pumped_flux_compression_generator
in electronic warfare, the natural dominant frequencies of an EPFCG are usually too low (they aim for 3 ghz and up, it seems) so they're connected to a vircator or something similar.