Hehe, I think the Mando helmet is easier to breathe in because it's open in the bottom. The vader helm is relitively closed, thus making it harder to breathe in.
I tested a Jango helmet at the Episode 3 launch here, it was very comfortable, not hard to breathe in at all. Think it mostly depends on how closed the thing is, and how you design it that counts in the end. That's why more personal are more comfortable, because you make it to your needs 
I'm currently sitting here by the compy, with reference and a cup o' coffee, and drawing the lines for a better picture. I'm getting the scanner tomorrow so I'm uploading by that time.
Oh by the ways, what materials do you recommend for beginners vaccuforming? I'm a complete noob to that, but fascinated by the idea of it.
And thanks for the tip! I'll look into the "netting" 
Thanks!