I'm growing in a modified DWC, which I call an SWC. The Grobot app will calculate your VPD, and you can also set the leaf temp, which you'll need to take with an IR thermometer. Usually the default is that the leaves are 2C cooler. TBH though, I don't really pay much attention to it these days, especially in hydro. What I pay most attention to is getting the humidity down in flower. So I use a 200W small and quiet dehumidifier, it makes the room warmer which I'd rather not but it's better than humidity. Currently I have a plant vegging in this very dry and high VPD environment, which is not supposed to be good but it's going gangbusters.
Regarding res temperature, mine gets pretty warm in the summer sometimes 29C, everything is fine. Cooler might be better but it did no harm. Having said that, I have heard it mentioned that warm temps have less oxygen, but that's why I do SWC rather than DWC. If you have everything else good, then a slightly warm res is not going to matter.
I suspect it's the leaf surface evaporation that cools the leaves rather than the ambient temp.