Honestly the WNBA argument is really strong. I believed Vlasic could dunk before I thought about that.
The fact that shes 6'4 is part of the reason she can jump 6'10, so her height in reference to her high jump is kind of irrelevant. If she was only 5'10, for example, she probably would not be able to clear 6'10.
This is actually a tough question, but given that only 4 WNBA players have ever dunked, it seems unlikely that Vlasic could do it.
Why can Vlasic get higher in the air than they can? Look at the BB gals; they have to have a lot of bulk on them to play the game. They are probably carrying 20 - 30 pounds more than Blanka (except Grinier (sp?), where the difference is even larger). The women's game is not a lot about being in the air, so the emphasis is not on staying lean. In their current body forms, I doubt any of those WNBA gals can jump 6 feet, much less almost 7.