Rather than come up with convoluted mechanics and systems, lets take it a back a step and have a look at it another way.
The biggest problem with multiple condition users, from a hardware perspective, is the sheer number of calculations being forced on the servers.Condition specialists want to be in a position to contribute to the fight but their damage is spread over a very long duration.
Instead of being susceptible to conditions like standard targets, HP sponge targets, world bosses, legendary and certain champions would have an extra function to their defiance buff. As it stands, defiant stacks are dependent on the number of players present. I’ve seen stacks go up to 70+ meaning CC’s are pointless unless they’re coordinated across the whole mob which is impossible.
What I propose is that degeneration conditions that can go up to 25 stacks, i.e bleed, torment, confusion would stack as normal but as soon as the cap is hit, a stack of defiance is removed. Given the speed with which bleed builds up in particular, this will make condition users contribute to the fight much more effectively than before as they would have to constantly rotate their condition skills to keep the stacks flowing and thus the damaging ticking.
To counteract this, the target would scale up as normal depending on the number of players in the event but the number of defiant stacks increase by 50%, although I could see this going up by 75% or even 100% if the stack removal was too efficient. The removal of the last defiance stack would have the same effect as usual, i.e. target is vulnerable to CC’s.
For vulnerability, I wouldn’t suggest much in the way of a changing anything. It already scales up the damage done to the target by 25% and given how many gw2 gold, that’s already a metric tonne of kittens being dropped. Poison and Burn don’t really need to be changed as their duration is generation very short.