The Question is:
Can you put a new video card into the unit if the unit has an on board video
The Answer is :
Generally yes, though details of the particular graphics controller, of
the specific VAX or Alpha system, of any other graphics controller(s)
installed in the system, and the number of available slots (if any)
would all be needed for a more specific answer to the question.
Multiple graphics controllers (of the same type) can operate in parallel
in what is known as a multi-headeded configuration using DECwindows.