Just to clarify something that was said earlier. When a tank is "vacuumed", (another way to put that is "a vacuum is pulled"), the majority of the gas which remains in the tank is removed from the tank via a vacuum pump. Vacuum pumps are capable of pulling deep vacuums and therefore leaving little or no undesired gas in the tank prior to filling filling with the desired gas.
This is what is done for the higher grade Heliums. The higher the grade, the deeper the vacuum that was pulled on the tank, the less residual gas was left in the tank prior to filling.
In our neck of the woods, Industrial Grade and Ballon Grade are the same thing. They say they don't add O2 to the ballon grade but who knows?
They don't pull the tank into vacuum for those grades, just bleed the tank to atmospheric pressure and fill. That means whatever gas was left in the tank at atmospheric, remains mixed with the Helium. Generally not a big deal in such small quantities.
But the only way to be sure is to get the higher grade Heliums. Considering the liability issues everyone is dealing with today, i wouldn't trust anything but the higher grades. Its not worth the money you save IMO.