Obtained: Speak with Bulma in Capsule Corp. during post game with Trunks after completing the sub story Lone Guardian
Objective: Join the others at Kame House
Head to Capsule Corp. and speak with Bulma to learn about a meetup at Kame House. Agree to go in her place to wind up there for a man’s night. When you regain control of Trunks, head back to Capsule Corp. and speak with Bulma again. This leads to a training session with Vegeta. Defeat him to complete the story.