Running VAX binaries per-se isn't a problem, you can run them on SIMH quite reasonably[1]. The challenge is running VAX binaries that use authorization keys (PAKs). Unless you have the key (or forge one) the binary won't run.
I have keys for the VAX assembler (and Fortran, BASIC, and Ada) so running source, and building applications that DECUS distributed back in the day, isn't a problem for me.
What annoyed me was that VMS/VAX support was discontinued by the hobbyiest program. When it was announced and I asked for details they suggested I contact licensing. Which I did and suggested I could buy new licenses for VAX machines for $5,000 good for up to 25 users. For a machine that was headed for the skip. So someone, somewhere, and I'm thinking US GOV here, "can't" upgrade their VAX hardware and HP has them over a barrel so they continue to hold those licenses as saleable products. But that is purely speculation on my part.
[1] In fact, for the earlier models (anything before the VAX 4000 series), they run faster on a modern PC under simh than they do on the original hardware.
MACRO32 is a VAX assembly compiler - given a VAX assembly source listing, it can turn it into object code for Alpha, IA64 or x86. In some cases, the code will need special annotations.
There's also VEST on Alpha which does translation of user-mode VAX binaries into Alpha binaries. I think that was an optional product. Many products (including things unrelated to the operating system) don't fall into the user-mode category due to the use of the "Privileged Image" mechanism of VMS.
Nitpick: Neither are emulators in the strictest sense of the term.
Where you have the source, you can build a native binary. Where you lack it, perhaps the emulation is sufficiently robust.
https://en.m.wikipedia.org/wiki/VAX_MACRO