But it may not have been voluntary. Heck, it probably wasn't.

It was voluntary, in a strange way. Another personality of him ordered his heart tore out. It's like the movie

(didn't read the rest of the thread, but felt the need to answer the OP)

My understanding is that Dagoth Ur's increasing power and connection/proximity to the heart, which allows his power to nearly overshadow the power of the Tribunal, also is allowing him to interrupt the reconstruction of the Tribunal, should they be killed.

Vivec states that it is taking nearly all of his power to maintain the ghostfence due to Ur's increased power. I thought it also stated somewhere that Ur's increased power might prevent one of the living gods from reconstructing should they be killed.

Ur never leaves the immediate vicinity of the heart, so when he is killed, he immediately reconstructs in the heart chamber.

Vivec, and the other members of the Tribunal, have been distant from the heart for so long that their power is slipping away, as such, they may or may not be able to reconstruct themselves after dying (and in Vivec's case, if you choose to kill him, you see that he is indeed unable to reconstruct at the heart)

Yeah, but this begs the question (at least for me anyway): how the hell would Nirn be able to exist without a Tower?

The reason Vivec can be killed easier than Dagoth Ur:

Bethesda hadn't yet mastered the "essential" flag.