Why don't they make combined (vacuum + flat panel) solar collectors
They do (for a good example, see TVP Solar - home). Flat plate evacuated collectors produce heat at relatively higher operating temperatures than the flat plate solar collectors typically used for water heating. They also collect more heat per square foot of gross collector area than evacuated tubes because they collect heat from the entire gross sun-facing surface area of the collector. Evacuated tubes lose the sunlight passing between the glass tubes unless the tubes are installed with focusing reflectors behind them (which can be problematic from a cost and maintenance standpoint).
However, evacuated flat plate collectors tend to be much heavier because of the housing and glass thicknesses required to withstand the atmospheric pressure trying to collapse the evacuated collector enclosure.