We already know that vinegar is a great weapon for fighting carpet stains. Adding a little bicarb soda can take it to another level.
Household blog One Good Thing by Jillee suggests soaking the stain with vinegar, then sprinkling in a little bicarb soda. The order is important here; pour vinegar on a pile of bicarb soda, and you have the makings of a carpet volcano. Once you have a bubbling mound of soda on the floor, just wait a few moments for it to absorb the stain, then wipe and vacuum it up. Jillee used this mixture to clean up pet urine, but it should work on a variety of stains, so hit up the source link for some more photos of this hack in action.
How to Get Rid of Pet Stains on Carpet [One Good Thing by Jillee]