Unix on a DEC VAX?

You're question is pretty general, so I'll just take a little stab at the NetBSD part:

The webpage of NetBSD's vax port lists supported machines (such as yours), many can use NetBSD 6.0.1, some are only supported in -current.

It also points to VAXarchive, a website collection some information that might help you further. It also points to the vax port of OpenBSD (supported machines includes your Vaxstation 4000/90)

Finally, both of these ports have mailing lists: NetBSD's is pretty active, OpenBSD's not so much.

Edit: here are a few historic UNIX releases that might be of interest...

My answer is to the "any suggestions" part of your question.

While you hunt down a terminal, note that if you have another computer with a serial port, you can use a null modem cable and a terminal emulator program to connect to the console. On windows you can use putty, tip on solaris, or minicom on linux.

(if you have USB but no serial port, you can use a USB-serial adapter)

Which Unix version to install on the VAX? Original UC Berkeley BSD Unix? OpenBSD? NetBSD? Ultrix if I can get a legal copy? AT&T Unix?

I'd go with NetBSD, but there is also a Linux port that may support 4000m90. There is no FreeBSD port.

Original (4.x) BSD does not have CPU support code for this model, and neither do Ultrix and AT&T Unix.

Is there a way to dual boot between Unix and VMS?

Yes -- keep them on separate physical disks.