Vima is a promising young padawan trained by one of the best and of a very high force potential.
Johun, on the flip side, certainly has more experience and is probably physically stronger.
The problem is, while we know Vima's talented and has good training, she literally never got the chance to fight anyone in her comic, just train with Ulic, and she's rather young, still a padawan.
However, one thing we can use, is the fact that Bastila Shan was originally supposed to be Vima. It's not canon-canon, but the author intent was for Vima to be that strong. And I'd bet on Bastila over Johun.