Those are rather warm temperatures, so probably most highlanders won't do well, although also most highlanders are rather large.
They appreciate a light, airy mix that gets flushed often. I just usually put some of this and that in there, but usually my mixes include some of the following:
Orchid bark
Peat
LFS (long fiber sphagnum (moss))
Horticultural charcoal chips (don't use very much)
Perlite (If I have it)
The stuff that makes up the bulk of the soil is the LFS and the orchid bark and if I have it, perlite. The peat is just to hold the stuff together, and it helps to hold onto a little more water. The charcoal is mostly just for drainage and it makes me feel like I'm doing more work.
So maybe like 2 parts LFS, 1 part orchid bark, 1/2 part peat, 1/8 part charcoal (as in like a large pinch for a 5 inch pot), 1 part perlite.
I'm not sure how your setup is, but I figure it would probably be best to have the Nep in a pot hidden in some moss or substrate, just to make it easier to take care of, and also, if the vivarium is not too small, you can take it out to water. Although, if you plan on just letting it grow wild in the vivarium, leave out the charcoal part, because it can apparently break down and get funky nasty. They like to be moist, not soaking. I tend to water whenever the top of the soil is crisp, then I pour lots of water over the pot, so that it gets lots of water. Although a humid vivarium wouldn't require frequent watering.
Neps tend to like bright light, and if we're going with fluorecents here, maybe around 80 or so watts, but that would be just if the plant is around 5 inches underneath, so more would be required if they are lower down.
-Ben