Performance fees in the second half are expected to be similar to the 1.2 million Swiss francs ($1.2 million) earned in the first six months of the year.
Money from brokers and independent advisers accounted for about 50% of the $225 billion Vanguard has brought in this year.
Investor interest in low-volatility strategies reached a fever pitch earlier this year, with $8 billion pouring into the most popular funds.
McNabb said investors pay an average of 12 cents per $100 for Vanguards mutual funds and ETFs and that in five years that number could fall below 10 cents.
Assets under management increased 13.6% as the firm brought in money to both its active and passive funds.