How does a company like Gilead get away with charging Americans all told over $100,000 for its hepatitis C treatment and through an agreement with a generic drug company (I presume a limited license agreement) make it possible for Vietnamese to get the same thing for around $6,000?

That the States permits this kind of obvious gouging indicates there is something seriously wrong with its patent and copyright monopoly approach.  A tenth the profit they are taking would still be enough to motivate the risk taking and study expenses.