How did Dooku give Pre-Vizsla his scar?
*serious picture*
So Vizsla said ""You see this scar? It was a parting gift from Count Dooku! If you question my resolve to destroy him again, I'll give you one just like it!"
Do you think Dooku tagged Vizsla out of spite? (very unlikely IMO as he's conservative with his saber)
Or was Vizsla dumb enough to engage Dooku?
The later scenario is more likely as I doubt Dooku went out of his way to physically meet with "some peasant". I'd envision Dooku screwing Vizsla over via hologram, Vizsla's forces flying out to where the separatists + Dooku were stationed, and surprising him with an attack.
I found this nice potential illustration of the fight:
If Vizsla actually managed to hold his own against Dooku it'd be both extremely degrading and impressive.