This chapter of the guide to Dragon Ball Z: Kakarot has a walkthrough for The Sins of the Father side quest.
Time frame: From Majin Buu Reborn - Episode 8: Finale to No time limit
Reward: 190,590 EXP, D Medal, Improved Scouter
Where to start: West Area, as Trunks.
To start this task, you need to talk to Bulma and select the "Find out what's troubling Bulma" option.
Your task will begin with a conversation in the Kame House.
To complete the mission, you must fight Vegeta. If you win, you will complete the mission.