Vertex Pharmaceuticals (Nasdaq: VRTX) saw sales of its hepatitis C drug, Incivek, drop off precipitously in the fourth quarter, year over year. The fast decline in Incivek sales, less than two years after its U.S. Food and Drug Administration approval in May of 2011, underscores the fast-moving competitive landscape for hepatitis C. Cambridge, Mass.-based Vertex and several other competitors are locked in battle to try to come up with an all-oral drug cocktail that would eliminate the need for interferon…