***WARNING: FULL SPOILERS FOR DRAGONBALL SUPER EPISODE 60 BELOW***
The newest episode of Dragon Ball Super revealed the identity of Black Goku this week, indicating that Zamasu used the Super Dragon balls to wish that he and the real Goku switch bodies, only to kill real Goku afterword.
From there it's gathered that the god king used time travel to make a second wish for his other self to become immortal, then killed to get the time ring, and so on and so forth.
Ending on a cliffhanger, previews offer that Vegeta is looking forward to go a second round with both fighters. So far, he still has yet to prove his fighting ability within the series.