Vertex Pharmaceuticals hasn't even received FDA approval for its second drug to treat cystic fibrosis, but investors are already weighing the impact of trial data expected this month from what could become its third. That's because the Phase 2B trial data for the drug known as VX-661, which Vertex has said it will present by the end of March, represents only the second time the Boston biotech has given any human trial data on the effectiveness of the drug.