Verdandi

I've also heard it pronounced ver-Uh-than-dee, with what sounds like a roll on that "r" and a softening of the first "d" more in accord with what I know of "ð". No telling if that's an accurate pronunciation.

At least in Sweden it would be pronounced vair-dun-dee.

As to whether it is pronounced ver-dan-dee or ver-don-dee really depends on the namer as it is common among Norse names, especially ones from mythology and other Germanic languages in general, that vowels are very open to change. You might even pronounce it ver-den-dee. Take a look at some other English, Germanic, Norse, Finnish names and look at their origin words and how the verbs are interchangeable.