Well,
Considering that I'm a 22nd level elven archer with a multiclass Cleric feat, what are my options?
Everything as long as you worship a god of the sun/light (amaunathor/pelor) whoever that might be in your world. The only exception is power of the sun b/c you've got no divine at-will and if you take morninglord that feat is worthless on top.
AFAIK, there is no way for you to add radiant vulnerability w/o rewriting your char.