The vpd chart is very useful if you get it. There was a thread about it just yesterday. But the ideal in my opinion moves throughout the grow cycle. Anything in the mid 70's F is good, too low and it's cold and not worth paying to keep it at a certain point. So for that temperature you need to determine how much draw you want the air to have upon the plants leaf surface. I like 1.15 to 1.20, unless I need to play catch up, but that's a different story. Anyways, back to the desired humidity. @ say 74 I'd like 60%.
Reason being @ 55% the plant needs a supplementation of water between scheduled watering/feeding. Like an unscheduled one or I need to increase pot size. There is to much drawn from the air, there is a lack of moisture and the plant had to make that up.
On the other end of things, if the humidity were instead 65%, the plant would not consume all that it is given and my run off would be greater. Because of this I would get a build up of moisture below the canopy encouraging anarobic fungal growth.
Now just a few degrees warmer and things change. @ 78F I'd wan't the humidity up at 65% to slow the plants rate of transpiration. Any drier and the plant needs more water and less nutrient.
I hope this helps to clarify vpd.
Now leaf curl, that's typically excess nutrients in the soil. Maybe it was too dry and you fed instead of watered.
Tell us more about your environment.
