PHILADELPHIA (TheStreet) --aOn Friday, the annual three-day meeting of the "Bogleheads" -- an online group dedicated to the low-cost index investing philosophy of Vanguard's founder and former CEO John Bogle -- wrapped up outside Philadelphia. After listening to Bogle speak, on Thursday the 250 Bogleheads eagerly filed off buses to Vanguard headquarters.a a That's right -- these investors love their mutual fund company so much that they spend their vacation visiting corporate HQ.