There never was a VAX64.  The VAX was itself a 32-bit extension of the 16-bit PDP-11.  Well, sort of.  The PDP-11 came out in 1970 as a 64KB machine (many had much less RAM, as core memory was expensive).  A few years later, they were already running out of address bits.  They expanded the PDP-11 in several ways: separate 64KB spaces for code and data (both for applications and system code), and a virtual memory system.  It was still a 16-bit machine, though, so accessing all that memory was cumbersome.

The VAX came out in 1977 (and probably didn't really work until 1977) and was a very nice 32-bit machine: flexible instructions, easy to understand if you understood the PDP-11, largely compatible with PDP-11 hardware.  It could even run (some) PDP-11 applications directly!

The VAX looked a lot like the PDP-11 (if looked at in the right way).  Enough so that assembler programs could easily be translated to VAX machine code and that compilers were easy to port over from the PDP-11.

In the 80's, Digital realized they were running out of address bits on the VAX but they didn't know what to do about it.  Their VAX machines were also slower and more expensive than the new RISC competition.  They spent a few years flailing about, going on all directions at the same time.  They probably had an internal project working on a 64-bit VAX (which we don't know anything about) and about a handful of different internal RISC projects.  They ended up making machines with MIPS CPU's in them + 32-bit VAX machines + PC's + PDP-11's.  And in 1992 their 64-bit Alpha RISC CPU came out.  It was fast, it was clean, it was expensive.  And it didn't really get rid of the VAX.  Or the PDP-11.

Commercial VAX emulators are still available (for lots of money), as are PDP-11 emulators, and Alpha emulators.

They probably could have made a nice 64-bit VAX instead of the brand new Alpha architecture.  And they could probably even have made a proper 32-bit PDP-11 back in the mid seventies.  Instead, they split their customer base and ended up having to support both the new architecture AND the old one.  Twice.