I honestly don't understand why it is so hard for some people to understand the fact that an ideal vpd will give your plants faster growth rates. Just because you are growing in a 30% RH environment, does not mean its ideal for your plants growth rate. When using a RDWC system correctly, it will increase growth rates beyond soil and other hydro systems, even with low RH, but if the vpd is in an optimal range and the other factors were dialed-in, then the growth would be even better.
Its like saying the world is flat, we all know science proved that the world is not flat, but since it looks flat with the naked eye, there were many people who said it was. Plants can look perfectly healthy when grown in low RH, but it does not mean they are growing as fast as they could be compared to a higher RH so the vpd is optimal, which increases co2 absorption. Not just cannabis, but lettuce and other plants have better co2 absorption rates when vpd is ideal.
This is botany 101, come on people!!!