Welcome to JustAnswer, and thank you for putting your trust in me!
I see that you've already tried using vinegar. Although this might favor a different approach, you should first make sure that you're using the vinegar properly for this purpose: use white distilled vinegar full strength, and soak your denture
overnight for a full eight hours. After taking the denture out of the vinegar, rinse it off, and then brush the stained areas thoroughly with a stiff brush. This may take a bit of elbow grease, but the vinegar should have softened the mineralized accretions sufficiently to ease the removal of stain. Be sure to brush vigorously-- soaking in vinegar alone will not do it; you must brush off the stain, and work to get the bristles into the grooves between the teeth and the pink denture base.
If the vinegar and brushing treatment still leaves stain, you may substitute laundry chlorine bleach (Clorox) for the vinegar-- dilute one part of bleach to three parts of water, and handle carefully because bleach is caustic and can cause chemical burns on your hands and damage your clothes-- if you have rubber gloves, you should wear them while handling your bleach-soaked denture. Again, leave the denture overnight or 8 hours soaking in the bleach solution, rinse off throughly, and brush any remaining stained areas vigorously. You should not use this method frequently if your denture has metal components, because it can cause them to corrode.
If your home methods are not sufficiently effective, you should bring your denture to your dentist
. He will have an ultrasonic cleaner with special chemical detergents that are more effective than anything you have at home. This will also allow him to check that your denture still fits properly and is not causing any injury to your gums and mouth.
Hope this helps...