What air pressure would be needed for water to boil at 80°c

The pressure needed would be of less than 1atm but not 0atm. For every 1 atm increased/decreased, the boiling temperature of water increases/decreases 20º C:    

                                                                                                                        ± 0 atm - 80º C                                                                                          1atm - 100ºC                                                                                            2atm - 120ºC

It's a very tricky experiment, since you would have to control and regulate pressure with extreme precision, and what is said above works better on theory than on practice.

Hope it helped,

BioTeacher101

beta decay is the answer