In my opinion, the short answer is Love.
Universal, eternal, undying love and compassion. Seeing as Vehk is "polysixual," it makes a deal of sense. I'll try to take a shot at explaining it.
Think about it: Anuiel basically needed Sithis to know who he was; as I've said, he said I AM ANU and I AM NOT PADHOME. In this sense they need the other to survive--no, EXIST in general. But, if they need the other to exist...wouldn't it....wouldn't it seem like they were the same thing? Wouldn't it be like they were indeed part of something bigger than themselves, something that eternally connected them, something everlasting and undying. As with Anu and Padhome, it happened with Akatosh and Lorkhan. Akatosh first said I AM and then Lorkhan said I AM NOT.
Lorkhan failed by his own devices to literally show us how not to (he didn't necessarily Zero Sum, but he didn't achieve CHIM, as we can see). Why would he do this? Love. To share his gift with mortal-kind, the Tower. The "I". The letter written in "uncerta
inty: CHIM.
Someone who knows Love realizes that he doesn't die. The only death you TRULY have is of Love, the death of the individual as he is conjoined with all of existence...yeah, I kinda jacked that from Albides' sig, but I feel it holds much merit nonetheless.
http://www.imperial-library.info/obscure_text/5th_era_loveletter.shtml
I honestly thought of some of this last night before I went to sleep, but I've forgotten exactly what I said (thanks to this damned headache), but that's the best I can do right now. And I really hope I didn't confuse you with my ramblings...