Well part is, and part isn’t in this case Chris. I always try to keep in mind a balance of salt and fat when they are together in a dish. For mac and cheese the high amounts of milk, cheese and butter means it often needs something to lift the heaviness of it and provide a more balanced flavor. The vinegar does that. You won’t notice it when you eat the final dish, but you may notice that the flavor feels a bit brighter overall. I’d recommend making your cheese sauce first without it, then adding it and see how it improves the balance.

Then for the Old Bay, yes, I did that just for a new flavor profile that compliments the seafood and it works really nicely here. –Rachel