You shouldn't need to cover the top. In fact, I'd advise against it. Keeping a level of air movement will help prevent fungus. I have the 3 gallon and humidity stays at 80% or so in an air-conditioned house just from evaporation from the soil. That's why the terrariums are bowl shaped (I think): so that the openning is small compared to the internal area allowing less humidity to escape.
Your plants don't NEED full sunlight, but I think VFTs are supposed to get something like 16 hours/day of direct sunlight (someone else can confirm for me). Don't forget to slowly acclimate them if they were shipped in (1 hour of sun the first day, 2 hours the next, etc). That will prevent them from going into shock and being burned. If your leaves start to look yellow or shrivelled at all I'd take them out of the sun for the day.
You may also want to look into just getting a desk lamp or something and a "daylight" CFL. It's not quite as good as sunlight, but it'll keep the plants healthy enough and you won't have to worry about finding a place for them. Email me for details on that, it cost me around $18 and is fine for our little 3 gal terrariums.
As for water, I've had it explained that the soil should be wet enough that if you pick it up and squeeze it, a little water should run down your finger. VFTs like it very wet. Try to keep it somewhat soggy, but if you see standing water I'd think that is too much.
Hope this helps. Hopefully some of the experts around here can confirm everything for you, but this should get you started. Best of luck with the new plants.
