For what its worth, I would check the datasheet for whatever device you are using to see what is connected to the paddle (the big square pad under the QFN package). Not soldering it down is not an option on some chips (indeed, I work for a company that produces such a chip, and we've had problems in the past where customers didn't realise they had to solder it down properly then wondered why the device didn't function correctly). On our device, the paddle is the analogue ground, without it soldered, the digital side of things works fine but as soon as you try and use any of the analogue functionality it does nothing!
The vias are there for a number of reasons:
1) They provide an escape path for vapours/gasses that occur when the solder melts during reflow.
2) They provide a wicking like action when the solder reflows, and this can help "suck down" the device to the board, stopping it from simply floating on the solder.
3) In the case of RF circuits, reduces impedances.
There are probably others too, but that's the reason we specify them to be there.