During the Dark Obsession mission, Vader gains the special rule
"When vader is attacked while adjacent to an imperial trooper who could be a target of that attack, that trooper may become the target of that attack "

My opponent is attempting to use a use cleave after hitting an adjacent storm trooper. (There are 2 stormtroopers adjacent to vader that could be the target for the attack by garkaan.

Cleave rule says from page 8 in the core book.

"If the target of this figures attack suffers one or more dmg, the attacker may choose a different hostile figure or object that he COULD target for the attack".

Can garkaan cleave vader even though there is another trooper who is a valid target for an attack? Or is vader immune to this since he by having 2 troopers adjacent is not a valid target for an attack yet?

Yes, he can cleave Vader as cleave is not an attack.

but cleave states the target must be able to be attacked, in this case vader is not able to be attacked while he is adjacent to those troopers. hence the dispute

I believe he is able to be attacked, he may just choose to redirect it.

The special rule for Vader is when Vader is attacked, cleave is not attacking Vader

During the Dark Obsession mission, Vader gains the special rule

"When vader is attacked while adjacent to an imperial trooper who could be a target of that attack, that trooper may become the target of that attack "

My opponent is attempting to use a use cleave after hitting an adjacent storm trooper. (There are 2 stormtroopers adjacent to vader that could be the target for the attack by garkaan.

Cleave rule says from page 8 in the core book.

"If the target of this figures attack suffers one or more dmg, the attacker may choose a different hostile figure or object that he COULD target for the attack".

Can garkaan cleave vader even though there is another trooper who is a valid target for an attack? Or is vader immune to this since he by having 2 troopers adjacent is not a valid target for an attack yet?

Remember, vader can be a target, but the special rule is giving you, the player, the option of making an adjacent figure the target.

So cleave could hit vader. Just because you chose to make the attack hit an adjacent figure doesn't mean it couldn't have hit vader in the first place.