Emacs on OpenBSD for DEC VAX

Are you sure it only lets you use one disk?

It asks you for a root disk to install the bootloader. You should select the smaller disk and select manual partitioning. Read the manual for disk label or, if you know what you are doing, the installer help.

After you set this disk up, other disks should be offered for setup in a list. Enter the name of the correct disk and follow on to disklabel and add the remaining partitions in that disk.

I've only ever used the emulated VAX in simh, but I doubt there is any difference as long as both disks are actually detected correctly.

Emacs can be relocated mostly harmlessly, even if you don't take any precautions when compiling. If the hardcoded paths don't work, Emacs looks for directories near the executable.

Emacs tries to determine where the executable that invoked it is located. It stores this information in the variable invocation-directory. Let's say that this is /path/to/bin/emacs; Emacs looks for the data files it needs in the hard-coded directories, and falls back to directories in /path/to.

You need to structure your directories in the same way as the Emacs source, more or less, with toplevel directories bin, etc, leim, lib-src, lisp, site-lisp. In particular, at least with Emacs 23.2, the directory lib-src must exist (even if it's empty).

There are a few directories that Emacs doesn't find this way. Set the environment EMACSDATA=/path/to/etc. You may need to set INFOPATH as well.