Very tough call.
I would initially say all of them would probably be within 5-10 points of each other during the season.
Vrbata probably has the highest potential only because he doesn't have to deal with depth chart issues as much as the other two. In PHX he really only has to compete with Doan and Sullivan for points... With that said, he doesn't have the offensive help as the others...
Berglund probably has the most talent, but he's probably hit by the depth charts the worst. At the end of the day, if you look at the STL depth charts, you have McDonald, Backes, Oshie, Perron, Steen, D'Agostini, Stewart, Langenbrunner, Tarasenko and Schwartz to compete with for points. If he could manage his playoff ice-time (20:08, 3:14), then a 65+ point season could certainly be in store for him, but if it reverts back to what he posted during the regular season (17:57, 2:22), I wouldn't be at all surprised to see a 45 point season from him... So he's completely boom or bust IMO.
Williams is probably the "safest" option, but his upside is limited. The negative is that the Kings are a "defense first" team. They'll win games 1-0, 2-1, so they won't post huge offensive numbers. Also with Kopitar, Brown, Richards, Carter, Penner, Gagne and Stoll, the competition for points is pretty stiff. He'll still get plenty of top line PP time alongside Kops as the chemistry between the two is pretty good, but I think the points will be limited this season.
As I said earlier I think all 3 will be relatively close to each other, so there won't be a huge advantage of 1 player over the others.
My preference would be:
Vrbata
Williams
Berglund