Tyvek is not, repeat NOT, waterproof. Somewhat water resistant, yes, but not suitable for wet situations, either as a ground cloth or shelter. You might try silnylon...extremely light weight, and waterproof enough to keep the rain out. Not much good for groundsheets, though...use 3-mil polyethylene or similar.

How about a Ray-Way style tarp & net-tent? You can pitch the fly as high as you want.