If were getting back to ops question: the whole idea of vpd is to maximize transpiration, or at least to get it to the point where its able to support the level of photosyntesis desired. in many comercial settings you try for 600ppfd even (40000lux iirc) and dialing in environment/vpd.
Remember vpd for low/no heat lights (leds) and IR heavy lights (hps) is not the same. Also remember you need a certain heating of the plant to maintain its metabolism rate in high intensity scenarios.
Another thing to take into account is light quality; amount and nm of blue and uv is also responsible for opening stomata which will also affect transpiration, a high blue light will better in part for this: its able to push the stomata open in low light intensity situations. People sometimes miss out on this with leds: in veg we want to give less total light (DLI) as the plants is not yet able to take very much. But the actual opening of the stomata is governed by ppfd of blue light (with certain nm being weighed more). So if your running low intensity with little blue then it can get very difficult to get the plant to transpire without giving it too much light for 18 hours.
So maybe i should revise my original statement: vpd is extra important when you are pushing the limits of DLI of what your plants can take, both in terms of long hours and high intensity. But if you go little by little with this sooner or later you get to a point where the plant is able to take massive loads of ppfd, this is when to really push it. Point is that you need to let the plant show you. Vpd in pure numbers are a bit pointless if you dont consider plant response. Point is: in a grow situation where you can deduce or feel that theres an environment related problem making the plants sag and not transpire, vpd will point the direction of where to take the environment, not an exact value. You slowly take it towards that direction.
Another layer of the oniion is CO2: high CO2 levels tend to close the stomata so you may need to compensate for this. Also, high CO2 levels + high intensity needs a few extra degrees of temps.
In the end the question is not is intensity or vpd more important; having reached basic levels of intensity (flower 600ppfd) most important is to understand how environmental factors, ppm in your feed, light quality all comes together and to be able to see and spot the signs of everything working well.
This is my fav vpd resource, it lets you set leaf temps as + or - degrees oof lleaf temps to account for different types of lighting. Even sso, it only indicates directions not absolute numbers imo.
