It should decrease, because according to the PVT relationship (e.g. PV=nRT), if pressure decreased, the boiling temperature should also be decreased.

It will increase For example the boiling temperature of water is 100 C at 1 atm And the vacuum pressure is the pressure less than the atmospheric pressure Like if you are at the top of mountain And you need to boil a water but the the pressure at the top of the mountain is grader than the pressure at the surface Then the boiling water will be less than 100C and the pressure is grader than 1 atm In this case the boiling temperature will be grader According to my example it will be grader than 100C My answers according to thermodynamics Table A-4 😉

it will rise because the vapour pressure becomes zero