I am not a big fan of buying tires at big box stores such as WalMart. Those folks are commodity sellers and their level of expertise is very low (as a general rule). If you have a problem their solution is to replace, rather than fix - and in the case of vibrations, sometimes replacing tires isn't what is needed.
As has been pointed out, the tires sold at big box sores are generally not the same tire sold at a tire dealer, so any warranty issues have to be handled at the same stor you purchased the tires from - and if asked, the tire manufacturer will direct you there.
And lastly I want to comment on tire returns. Far and away, most tire returns are for vibration. These are not "defective" in the sense that there is something in the tire that shouldn't be there - or something that should be there, but isn't. No it's more about degree - generally of uniformity (think "round" and you'll be close).
Vehicles have differing degrees of sensitivity to vibration, so it's difficult to set a spec - and the tighter the spec, the more difficult it is to manufacture a tire to that spec.
Some tire manufacturers measure every tire and some only sample.
So when it comes to big box stores, I tend to stay away from those for pretty much everything - although in today's environment, it's hard to avoid. Lowes and Home Depot have pretty much killed the local hardware store.