Purpose of adding a "sour" to the final or second to final rinse is two fold. One the acidic bath will remove the alkaline (base) residue from powdered detergents which tend to be alkaline in nature. Removing deposits of soap/detergent scum, washing soda, borax and such will result in softer and whiter/brighter laundry.
Secondly the acid will help restore laundry to a skin friendly neutral pH after an alkaline wash bath. This results in greater comfort for the wearer of the laundry.
If using a liquid laundry detergent, a sour really isn't needed as they are neutral to slightly acidic in nature due to their different chemical make up than powders. For one liquid detergents do not rely on washing soda and or borax (both are strong alkalines) as builders. Most liquid detergents use sodium citrate as a builder/pH adjuster.
The new fabric softeners in the market like "Downy Advanced" rely on cationic surfactants (a type of detergent) to soften laundry instead of the old wax and oil bases found in older formulas of FS. If you notice when a front loader starts to spin out the rinse water to a final rinse where "Downy Advanced" has been added, the water slighly foams as it comes down the window.
While cationic fabric softeners don not coat fibers the way wax/oil based ones do, they still increase the risk of fire since they make items more fluffy.
Many of the "green" fabric softeners are cationic based,including Ecover (my fave for the scent). In this way these products can claim they are not made from animal by products (fats, oils and waxes).
Usually add about 1 tablespoon to the final rinse water (in the dispenser), with or without a dab of Ecover FS. If your laundry smells like salad dressing coming out of the washer, or worse the dryer, you are using too much vinegar.
Finally it goes without saying, but is worth repeating DO NOT ADD VINEGAR TO ANY WASH LOAD THAT CONTAINS CHLORINE BLEACH. If you used chlorine bleach in the wash, it should be mostly if not entirely rinsed before the final rinse and you add vinegar. To an extent vinegar will remove/neutralise any chlorine bleach residue, but not wash water saturated with bleach.
Oh yes, you do know only to use white vinegar, right? No apple cider, flavoured or coloured vinegar as they may stain.
Launderess