They take the highest value from reputable exchanges, given their volume of sales they can likely lodge an automated trade very quickly and have minimal risk of fluctuation.
I would hazard that they make their money of large business / enterprise. And for small business off interest on money held for payout. Sure VC probably pays alot of their starting costs for now, but they do have sources of income and that's what matters in the long run. When I signed up they charged 1% and I was fine with that, but after splitting all the coins it probably dint work out very profitable for them on small transactions.
By the way to those who question the rates, or suspect a bit of money making off them. From the bitpay site:
"Buyers will always get a better value by spending their bitcoins at a BitPay merchant than by selling them on an exchange."