Berner's correct with the VT + MS method. In a case where there are 4-5 targets it is more powerful (assuming they live long enough for VT to tick it's full duration). To see the effect (and difference) check out parses of Anub'arak hard mode kills. With VT on the adds + MS I'm doing around 12k DPS, but when I have to pay more attention to VE heals in P3 and can't afford GCD's to put out VT anymore my DPS drops to around 8k with only Mind Sear.
There will be a point where VT is no longer worth applying though. Probably when there are around 7 or 8 targets.