Every investor has heard the phrase "buy low, sell high." It's a simple enough piece of advice that's less simple to apply. What exactly qualifies as "low"? One possible answer is when a company's shares fall significantly in one day following a setback that barely affects its long-term prospects.That's what recently happened to Vertex Pharmaceuticals (VRTX -0.46%), a biotech company. Here is why now is as good a time as any to "buy low" when it comes to this stock.Vertex's pain medicine underwhelmsVertex P ...