Oh man, this is such a great question CO2, VPD, and such. In fact, I’d call this quite advanced. Doubly so for a sealed room.
Certainly, look at VPD first, then worry about the addition of CO2. But, like Shady says, CO2 during lights on. Its part of the photosynthesis cycle (carbon fixation). No lights … no photosynthesis.
For VPD, the easiest way is to refer to VPD charts. You’ll need to be able to measure temperature and humidity.
Simply, there are “optimal” bands where the plant will excel by optimizing transpiration (exchange of moisture and carbon dioxide). The plant responds to the environment by opening and closing stomata that allows the exchange of CO2 and moisture (mostly). If the conditions are “off”, the stomata (pores) will close and limit several things in a chain reaction. By closing the stomata, the plant will slow the release of moisture into the surrounding atmosphere. Because the moisture is no longer being released from the leaves, the plant slows the uptake of nutrients from the roots. Closed stomata will also limit gas exchange which means the plant will not be able to perform carbon fixation efficiently. Without the necessary carbon and phosphorus (nutrients), photosynthesis stops or is significantly slowed.
By getting VPD optimal along with sufficient light and carbon, you’ll get the plant into the optimal growth mode.
This is a good one on VPD and the VPD charts:
And, source code and technical details for geeks (shameless OG plug):
This stuff is especially pertinent for a sealed room. Humidity and temperature may be limiting factors…
p.s. VPD charts are guidelines not rules. Adding CO2 will likely shift the chart somewhat.
Also, see this: