Your vet is apparently unaware that UTHs are not designed to increase ambient air temperatures, so of course it's not doing anything to warm the entire inside of the tub. As long as ambient temperatures stay >78, that's perfect for BPs. If yours is 80, then you're golden. As far as the hot spot temperature with the UTH, honestly you could probably stand to turn it down a few degrees (around 90).

Now, you mentioned nothing about the humidity. Do you have any idea where you're at with this? How are you measuring/controlling humidity in your tub?