In a sink or tub mix 1 cup of vinegar with 1 quart of water. A knack would be to test the mixture on an inside fold initially to avoid further damaging the item. Immerse the stained garment in the solution. Once it has absorbed the mixture, rub the soiled portion against the garment itself to ease up the tough stain and make it break free from the item. Leave the garment to soak in the vinegar solution for up to one hour. Lastly give it one wash in the washing machine separately or rinse the item thoroughly with hot or warm water to get rid of the vinegar odor and finally the spot itself. If the soil persists, repeat the above steps until you get the desired result.
A pro-tip would be to avoid overloading your machine and leave space for the churning and turning of clothes so that even the detergent gets to work its way in and out of all the garments.