I have both the S6 and T4. The S6 is pretty quiet, but can get a little louder than the T4. Depends of course if you’re running filters on the ends of them or what. I’d say the S6 on speed 8 or 9 sounds like the T4 on speed 10, so close.
VPD is Vapor Pressure Deficient and in layman’s terms describe the environmental conditions in terms of how hard the plants will have to work to transpire or conserve moisture. If you keep the VPD in an appropriate range, the plants are basically much happier because they not working as hard to maintain the environmental conditions the require. Here’s a couple of charts I use and and link to a much more detailed explanation.