I was playing and Vader had a stress token. He then recieved a critical hit whose effect said to immediately receive a stress token.

Does he get two? Do they stack? Or would the fact that he already had one mean the crit was kind of worthless?

We assumed he could have two at a time and would just have to perform two green maneuvers to lose them.

Your assumption was correct.

He gets 2 stress tokens, and must perform 2 green manouvers to get rid of both of them.

He also can't perform actions (ouch for Vader) or perform red moves untill he has rid himself of both of them.