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
