>> -(defun vc-find-revision-no-save (file revision &optional backend)
>> -  "Read REVISION of FILE into a buffer and return the buffer.
>> +(defun vc-find-revision-no-save (file revision &optional backend buffer)
>> +  "Read REVISION of FILE into a BUFFER and return the buffer.
>
> Since you use BUFFER, please drop the "a" part.
>
> Also, the doc string should tell what BUFFER defaults to if omitted or
> nil.

Updated.

>> +  (let ((filebuf (or buffer (get-file-buffer file) (current-buffer)))
>                      ^^^^^^^^^
> I would use bufferp here.  What if someone calls the function with
> something that is not a buffer?

Added buffer-live-p.

> And, btw, are buffer names allowed?

I see that most VC functions work with buffer objects only.
So buffer names could be allowed only when really necessary.

> Similarly with other places where you make the same test (why not make
> it once and bind some local variable to the result, btw?).

Done: