Planet and Tower vendors have had the same guns for weeks now. There doesn't seem to be much of any change in power level the engrams drop at either with me having all 335 characters.
The inventory is not set on a rotation like it was in Destiny 1. The vendor inventory is static with the exception of the weapon offered which rotates every so often.
As far as the level of engrams, it will fluctuate hourly even. There have been several times where both the vendors and/or Banshee were at 330 light, but it is rare and few and far between. This is intentional as the way to truly earn higher light gear is through completing all the weekly milestones.