Purple Sun is a vortex, not Direct Damage or Magic Missile, so no Vassal for that.
Vassal does, however, make Fate of Bjuna more viable, and better than the other snipes. Death also gives you Soulblight and Doom and Darkness, which are amazing spells, especially with salamanders.
Remember you dont have to sit in Temple Guard if theyre in your army. A small unit of Saurus or moderately sized cohort can give your slann cannon/template protection with less worry of killing the unit from the Sun. The sun is an excellent counter to Daemons, Ogres, Trolls and the Dwarfs that will be appearing in droves soon (potentially).
If youre thinking of running a Death Slann, i think its less about how to kit the Slann and more about how to kit the rest of the army.
Flying skink is good, but a couple of normal priests is fine i think. Theyre fast enough to get into position without fly, and can remain with a unit. Also gives you two Iceshard Blizzards, which I think is essential due to the one combat debuff your slann will/might have.
I'd aslo consider Bastiladons with the Solar Engine. A good, cheap bound spell that compliments death well, as youre forgoing the magic missiles from other lores.
