Hey man
If you want a terrarium than go for it. They seem to get a bad wrap on this site. I like terrariums just because they look nicer and have a more natural feel to them...i think theyre more interesting than a pot plant anyway. And by terrarium, im refering to a fully established environment, not just some plants in a pot, sitting in an old fish tank. If your going for venus fly traps id recommend a rectangular glass tank, however big you want, but with enough room for them to grow and spread their roots. By using a rectangular tank you'll be able to reduce the humidity alot more, and if you leave the lid off completely it will be barely different from the outside humidity. If you can, try and use artificial lights as this will reduce the heat and are less harsh than the sun. A mix of both is good, even if its not direct sunlight. As for the soil, if you dont want to spend very much money on fancy drainage systems, then dont, its not necessary if you plan your soil right. Id recommend using propogation sand (quartz/silicon based, make sure it does not contain salts!!) and mix that with some stones/pebbles (preferably quartz, dont use marble stones) just for the bottom layer to aid in drainage. For the top soil use a mix of peat moss and again sand, you could use perlite instead of sand. Mix it about 5:3, whatever looks right. Before planting your VFTs, give the soil a good soaking. Id also suggest adding live moss and maybe other small types of vegetation, maybe shammrocks (clovers) once you get it setup? This will help get rid of excess water, help establish the environment and make your terrarium look a bit more lively. I think thats about it.
Make sure you:
-use distiled water
-give them enough sunlight
-keep the air flowing inside the terrarium (take/leave the lid off)
-avoid over watering
-dont let it get too hot or humid (with a lid off or some ventilation gaps this should be fine)
I currently have some heliamphora minors and VFTs together in a terrarium (which i probably wouldnt recommend mixing) and theyre doing fine and theres been alot of new growth since ive gotten them. Mine are in a fish bowl :\ I havent been doing this for long but its not exactly rocket science...theyre plants