Vinegar didn't work to clean fire king measuring cup, what can I use?

Please provide more details as to what it's dirty from/with. That will impact cleaning suggestions.

I've had good luck mixing water with a little Comet Powder in it. Let the mixture set in the cup for a day or so, then wash with warm water & dish soap. You may have to do more than one treatment.

try barkeepers friend in liquid or powder it will not scratch the surface but does a great job cleaning

I use denture cleaning tablets to get stains out of coffee mugs. Just use water from the hot water faucet with a tablet and let it soak for a while.

I have tried the full strength vinegar in the glass dish and put it in the microwave till it boiled and then let it cool before cleaning it. I have cleaned the microwave that way and it works well. ? Why not give it a try? Teresa

I collect old Pyrex, and am in a group where people have suggested cleaning old dishwater damaged and baked on food dishes by spraying it in oven cleaner, and then putting it in a sealed bag for a few hours. I've seen pics of the results of this and it really works!