Be adamant, Missy! Tomorrow, say "I know this might seem picky, but wtice I have asked for oil, and gotten vingar, so I cannot eat the sandwich. Can you make extra-sure to use just oil today? Thank you so much." And then on future days, you can "joke" - "Remember me? I'm the 'oil - NOT vinegar' lady!"

In my case, as I mentioned, I do have food allergies. If someone puts a lemon in my water, I have to send it back. It's better than losing hours of my day being ill and in pain, and people are usually pretty nice about it.