I would think that a Viv would be way too humid for any grass other than a Bermuda mix or something similar to grow in. As far as for a burrowing frog tank, the grass would grow a root "mat", preventing the frog from burrowing if grown extensively enough.

Gail, there are plenty of options to have a grass like plant in a Viv. Since you're trying to create humidity, we can rule out any marginals, like eleocharus parvula. You can look into a few species of horsetail fern. There are a few species that are smaller in stature and would look like grass, all while helping keep humidity in the tank