During lights-off the stomata close up, so it is less important during that time. Transpiration slows in darkness significantly.
It is technically more stressful to have larger swings of VPD, but i doubt you'll notice a difference with the naked eye and small sample sizes.
could set it lower at night... bet the temps drop too, eh? so it doesn't take the same RH to maintain a similar VPD as you do during the day... VPD should be yur guide. temp and rh should jive together.