A couple of thoughts -
1) I've softened the veneer only twice - once when it was VERY rippled/wavy and once when it was very porous (I think it was birds eye chestnut) and I wanted to limit glue bleed through. 10 minutes was long enough in both cases.
2) I only use a lower platten only when veneering on both sides (balancing veneer).
3) Process I use - a) roll on glue with a wallpaper glue roller, b) place veneer, c) cover entire surface with wax paper in case there is some bleed through, d) position upper platten with slots facing upwards, e) slip into bag, positioning the "sandwich" so that one of the cross slots is directly under the vacuum nipple, f) seal bag.
4) for the upper platten, I actually have a set of different sized MDF that can be used in various combinations to work over many different sized stock. As Robert said, the slots are merely 1/8th inch deep crosscut spaced on inch grids to allow the air to be evacuated over the entire surface without forming trapped pockets.