I don't know about the venogram. I do know that every surgery and procedure I have had has taken a bit more of my remaining kidney function. I don't know if it's the anesthetic or the dye, but my ability to urinate lessens with every dose.
The only response I've ever gotten to my concerns about it is - "But you are already on dialysis. What difference does it make?"
Did they offer you any other options to make the fistula work? As frustrating as losing the kidney function is, having a working fistula, and not losing the one you have still needs to take precedence. There are only so many places to put them, so if it can be repaired and made usable, that should be the priority.