I decided to run it on SIMH which is an excellent collection of software aimed at preserving computer history through virtualization of obsolete machines.
I must admit that, for me, VMS brings about a bit of nostalgia, since it's where I cut my teeth. Some of the first systems I was ever lucky enough to play with when I was young were VMS systems and it inspired me to learn more and more about software and computers in general.
What I did here uses the latest version of SIMH built for PPC. You'll need to change SIMH a little in order to get it to build on the Mac, the instructions are here. Also, you'll need to follow Phil Wherry's excellent guide on getting VMS installed in the first place, that's here. And, finally, you'll need to get DECWindows installed, that information is provided here.
Information about the VMS Hobbyist program is available from HP and from this site. The only issue I found was that it takes a while to actually get the software since they want to first make certain that you have the membership so that you will not use the copy of VMS that they send for commercial purposes. :)
A fair amount of free and open source software is available for OpenVMS. I'm currently working on installing the latest version of GCC for VAX on my virtual VAX here.