What VPD actually describes
VPD is how strongly the air pulls moisture out of the leaf. Too low and transpiration stalls; too high and stomata close to protect the plant.
Targets by stage
Seedlings prefer a gentle pull, vegetative growth tolerates more, and late flower benefits from a drier room to protect bud quality.
Adjusting in the real world
You have three levers: heat, humidity and airflow. Change one at a time and give the room a full cycle to settle before judging the result.