Jeff Tjornehoj, senior research analyst for Lipper, cautioned that Vanguard's fund could lose out should Treasury yields rise sharply and unexpectedly in the future, though he said yields are unlikely to jump any time soon given the uncertainty about the euro zone and the global economic outlook.
Those resources have been stretched to the snapping point by the IMF's having to jump in as a junior partner in the EU's loans to the euro zone's weaker states.