Based on some of the responses, it seems that salt and vinegar chips are a relatively new thing in the US. Is this accurate?
Here in Canada, I've been eating them since I can remember. I know that its uncommon in the US to put vinegar on french fries (or freedom fries if you prefer).
Here you can get little packages of vinegar at McDonalds for your fries, but you need only to drive across the border 50 miles away to get blank stares if you ask for vinegar.
Once in MSP
they offered the bottle of white vinegar they used to clean the grill...
Although I love them, I have to agree with one other poster that they really irritate the lining of my mouth if I have too many. I thought I was the only one!