I’ve noticed my Kirkland brand cetirizine smells like vinegar. Is it bad?

Not necessarily. Vinegar contains acetic acid, the source of its odor. Cetirizine is 2-[2-[4-[(4-chlorophenyl)-phenylmethyl]piperazine-1-yl]ethoxy]acetic acid, which is an acetic acid, so it should smell similar to vinegar.

I've never bought a bottle of cetirizine, so I don't know what it smells like in quantity. However, I can't imagine that any drug manufacturer would design their pills to smell like vinegar (most of the tablet is filler or other agents, not the drug), so I'm guessing that your meds have a problem.

If you bought it at Costco, I'm sure they'd be happy to replace it. If you bought it on Amazon, you may have to call them, rather than use the standard return process (some things on Amazon are not returnable, in theory).