It's a good thought, but unless you have a way to maintain uniform thickness of the cast, your vision is going to be super distorted. I've seen a couple very nice looking vacuformed ones, and I'd have to say that's the best route to take.
I'm not sure if he is on the 405th, but I'm following a guy named roshprops on instagram, and his eva helmet casts and vacuformed visors are absolutely stunning.
I don't have any personal experience with vacuforming, but I believe getting a good result is a combination of having a very precise buck, an adequately powered vacuum table, the proper material thickness, and correct temperature of the material to be formed around the buck.
If you're skilled enough to create a system for injection or blow moulding, it's possible you could create a cast piece. Otherwise, I'd stick with vacuum forming your work - it's cheaper and there's far less to go wrong, which leads to arguably more reliable results.
That could be done, but the process is ultimately much more difficult than just vacuforming. I did it with a cheap homemade vac table, if you have a shop vac already you could do it for less than 200 dollars in materials. Follow the link in my signature to see the process and the results.