Flueless gas stoves are very efficieny, as another poster pointed out, you have no heat loss via the chimney.
These fires work as they have a catlitic converter, this scrubs the CO from the flue gases, the 2 vents one high one low are for the safe operation of the appliance and are required, make sure to place them in a place where the draught will be unlikely to reduce room comfort. Do not be tempted to only put in one as per regs from another country cos when your gas serviceman arrives he will be cutting off your appliance at your cost (he has no option but to do it so dont blame him either)
One thing to bear in mind with these fires is not to use ligthed candles in the same room as one that is operating as these can effect the coverter and prevent it from scrubbing the CO (a CO alarm is good advice)
Maybe another alterative, if the wall where your intending installing the appliance is an outside wall, could you not fit a low level balanced flue room sealed stove (no chimney and no extra vent required!)