I use a UTH underneath (outside) wooden vivs and they work well as long as a) the weight of the viv isn't resting on them and b) there's an air gap over the UTH to prevent overheating.
I set this up using cork floor tiles. I tape them together to make two mats, the size of the base of the viv. I cut a hole in one mat the size/position of the UTH. I put the complete mat on the floor, the second mat on top of that, fit the UTH into the gap in the top mat, and place the viv on top of that. The weight of the viv rests on the cork mats and the UTH has a slim air gap over the whole surface.
The mats are on stats and they heat the floor level - through melamine and wood - perfectly. If you have a taller viv you might want to put in a lamp or ceramic to heat the higher levels, but if this is on a stat as well, you should be fine.
I've had a bank of three vivs working with this heating system (UTHs only) for around six years now and not had a problem with it yet.