MSCDIRECT has prehardened ground steel stock. You should be able to find a size which will work. It can be machined!
Ok.. it's not as hard as the originals, but it should work.
As 'TDegenhart' says, follow his lead. He's right on point. I would add, 7075-T6 aluminum is HARD! (for aluminum) Your special jaws made for it will last a long time.
ADDITIONAL Information:
There is nothing wrong with a Bridgeport vise! Ok.. it needs some tuning up. A very good lesson for you is take it all apart and see how it works. Then either the front end or the back end could stand to have (maybe) an inch sawed off (1/2" off both ends?) to allow more open width. I have not done it myself, it's just an idea. (edit: I just took another look, the jaw probably needs to be that long to hide the screw.)
Also, under the moving jaw are side plates, they hold the jaw down, and were never all that accurately made. Examine them and figure out what you can do to make them a closer running fit. Examine it during the day and try to figure it out. But! Sleep on it during the night, and the next day your subconscious may have unwound a better solution for you.
If you can improve these conditions you'll have a better vise than the Kurt. You already have the swivel plate.
I urge you to give this an effort. A machinist is a machinist, because if he needs a tool he will make it, to solve the problem.
So if you need a better vise and you can improve on the one you have, and do it to your satisfaction, you will have what you want and you will also gain a LOT of self respect. You might even jump up, and throw you fist in the air and say, "I did it!"
Now the great news. Every time you use that vise, you will crack a smile of accomplishment. Believe me, there is nothing like that!
Regards,
Stan-