The best vectorscope for audio is the NTP in the CRT Danner cassette that was in the Neumann consoles. They have log amplifier inputs so they scale audio nicely.
Thanks Paul. I hadn't thought about using a log amp for X-Y display but it seems like a very nice touch.
I think old CRTs are going to be very difficult to source other than salvaging them from old 'scopes bought on eBay.
The CRT is only a couple of inches around. The image has to fill the screen no matter the level because there isn't much display room. I have the documentation around somewhere. I'll see if I can dig it out.