What can I make Vegeta's armour out of?
I was thinking Cell saga.
It has to be affordable but still good!

Finding a Pepakura pattern might be useful - pretty much origami patterns that can be folded into armor and rienforced - it's great for beginners to the armor scene!

Another option to try is Craft Foam or Worbla.
Craft foam however is very easily destroyed, and my first supply ofWorbla hasnt arrived yet for me to give you a critique, but I've heard it's EXTREMELY reliable and easy to use.

I think you'd get on well with EVA foam floor tiles they're easy to cut with a knife or scissors and can be shaped with things like a dremel hobby tool and can be glued together with a glue gun.

I prefer working with fabric as I'm rubbish with plastazote or worbla so would suggest make a vest base out of cheap white fabric. Adding muscle details using cushion foam and covering the pieces with stretch PVC fabric and sewing to get the shape