Fic ideas: 
Option 1: Vortex has a thing for First Aid, and he uses "kindness" to convince First Aid that he is the only mech who cares for him. He does this by  mechnapping First Aid and making him watch "Live footage" of his brothers having fun without him.
     A) It works, First Aid develops Stockholm Syndrome and believes he is "in love" with Vortex
          a) When they finally rescue First Aid he is carrying.

     B) It doesn't work because Vortex doesn't realize First Aid is bonded and he can feel his bonded coming for him. He let's Vortex think it is working in order to distract him.

Option 2: Vortex is genuine, or thinks he is, in his affections and separates Aid from his brothers every chance he gets so they can "Go on dates." First Aid is naturally freaked out.
     A) Aid's brothers don't realize what is going on, and First Aid is afraid to tell them.
            a) After "dating" for a few Earth months, Vortex tells Aid that he has been patient in their relationship but it is time for them to move to the next level...
        
     B) The Autobots find out and tell Aid to go along with it in the hopes to make Vortex switch sides. Cuteness develops.