What I've noticed is, that 90% of the time raid han stuns Vader in my team... occasionally he selects someone else. He does, however, go after Vader to an almost predictable level. When I fight a team w/ raid han, I either change my strategy to the expected stun, or bring in a pretaunt tank to limit the effect of Vader being stunned. Every now and then, I'm surprised & he picks another character... but it is rare.
This leads me to the most logical conclusion that the devs implemented a widely used AI method.... weighted decision-making.
Like I said before... It's not sorcery... It's not even an obscure concept. It is literally used in all decent AI implementations.