I agree with most of the comments. I grow VFT in New Mexico, US, where the humidity is rarely higher than the mid teens percent, and sometimes down into the single digits. I do nothing to intentionally increase the humidity in my VFT growing space, although in the summer we do use an evaporative cooler in the sun room / greenhouse.
When I place my VFTs outside to feed (we have trillions of billions of dairy-generated flies around here), the only precaution I take is to keep them out of the direct wind, which on these grassy, naturally treeless plains is often strong, hot and very, very dry. Other than that, the low humidity doesn't bother the VFTs at all (I do let my new ones adapt gradually to lower humidity though, and try not to stress them too bad all at once), and they are all not just surviving but thriving. The idea of planting a VFT in a terrarium seems odd and unnecessary to me personally. If I can grow them here where there is almost no humidity, then people ought to be able to grow them almost anywhere (except antarctica, perhaps ) if the soil is kept moist, right?
--Just my small addition to the discussion