Whats with the Damage % affix on items are they not wanted? Are they no good?

also

Critical Damage vs Vulnerable Damage whats better or are they both good?

They're not all that good. Like for example on a ring your way better off to get crit chance/attack speed/main Stat or life than any of the %dmg mods.

As far as crit dmg vs vulnerable it would depend on your class/build, if you have high crit chance probably go for crit dmg, or is you constantly make everything vulnerable maybe go for vulnerable dmg. But honestly those % dmgs aren't important enough to really stress over.

Wow thank you my friend really broke it down for me, appreciate you