Semi-depends on your raid comp. VT on everything will net you MUCH more overall damage. However if your raid is lacking burst dps to kill the closest adds in time, MS/MB would be the most beneficial
I usually tab VT all the adds and than SW:P / DP -> MB/MF the add that needs it. Optionally if you have good death grips + knockbacks, the adds can become clustered enough to Mind Sear. It is also good practice to time a Shadowfiend.
Edit: I just realized that I may be talking about the wrong adds :-x I was talking about Sons of Flame. For Molten Elementals, we aoe them down.