"That's true," Vash says.
He reveals that he was a bounty hunter for a short time. Rand gathered a few bounties in his time as a notorious Dark Jedi, so Vash pursued him. He confronted him in the Outer Rim, when Rand was running an information gathering mission for the Mecrossans about a potential ally. Vash wrecklessly cut his way into the meeting and actually had no idea that Rand had Jedi training.
As such, the duel played out with Vash initially losing his edge and fleeing. He lured Rand out into the streets, where the duel resumed until local law enforcement showed. It continued over the low rooftops of this particular city; Vash utilized his agility to outpace Rand, but Rand predicted his movements and neutralized the play.
Ultimately, Rand threw Vash through a wall with the Force, then convinced him to surrender and join him.