So after a week of trying and cca. 300 soul gems after, Voriak Solkyn is down!
My first vMA clear and I'm so happy that I had to share.
Did it with my templar healer (switch to dps for the arena).
Many thanks to all of your guides and comments across this forum, especially Joy Division's guide, @Lukums1
and to someone who suggested to use prismatic onslaught glyph on destro, I think that helped a bit.
It felt so easy when I defeated him that I really don't know how to hell I couldn't do it before. Guess practice makes perfect.
Now to see a shrink