Interesting...I posted a thread about my interest in vivariums a few weeks ago.
I have a 20gal tank setup and I use a humidifier thats on a timer to regulate the humidity. It help alittle with temp, but I find the temp better controlled by keeping the entire room cooler/warmer. The light is also on a timer and it affects temp very little. I drilled a hole into the bottom of the tank and a hose from the humidifier enters there. I also have a corresponding hole drilled on the other side of the bottom for drainage. All my plants seem to be doing rather well with 84-95% humidity (moss is staying nice and green) and temps in the mid to low 70's. I think Im about ready for some dart frogs.