Best soft waterproof bag I've found has hard molded bottom with a vent, $50 Amazon Prime:
https://www.amazon.com/gp/product/B00644OLLU/ref=od_aui_detailpages00?ie=UTF8&psc=1
I have that very same bag but in the XL size (110 Liter). Sits long ways on the tail of my for all my trips now (see photos in my flickr albums... link in sig).
Yes, it does have a rigid bottom tray that helps it to keep it's shape.
NO, THAT VENT DOES NOT LET AIR OUT FROM INSIDE THE BAG. The bottom tray is a separate additional layer that the bag envelope is attached into. That vent only allows air and moisture to escape from between the layers in the bottom. The vent doesn not go through to the inside of the bag.
When I roll down my dry bags (I use 2, this one and a Sea to Summit 35 Liter bag for my tent and tarp), I sit or kneel on them to squeeze as much air out as possible and then roll down the top. Roll once and continue to squeeze more air out, and continue with the second roll/more air, and the third when usually it is sealed and no more air can be pushed through. I try to roll it down 4 times if possible. Never a problem with the bag bloating from air inside heating up. Haven't really been up that high to see significant air pressure changes but heat from the sun warming the bag will also cause expansion of any air inside.
Love this bag. Can't be beat for the price and performance.