It would appear that you misunderstand the architectural difference between
VAX and Alpha. It is not merely a matter of "32-bit" vs. "64-bit". Indeed,
there is no such thing as "32 bit mode" or "64 bit mode" in OpenVMS.
A VMScluster may have a mixture of Alpha and VAX systems in it. The Alpha
systems must boot from a separate system disk than do the VAX systems.
For more details, please refer to the VMScluster Software Product Description.
Depending on what users do, the Alpha may appear as "just another VAX" (albeit
a faster one), but applications are VAX or Alpha specific - a VAX program
will not run directly on Alpha, an Alpha program will not run on VAX.
There is an optional (inexpensive) layered product called DECmigrate which
can convert many VAX executable programs and shareable images to run on
Alpha. This product is intended for use with old applications which you
can't recompile (perhaps you don't have the source code). Otherwise you will
need to recompile VAX applications on the Alpha system, using native Alpha
compilers and the native Alpha linker.