I, too, never heard of the Fabric Softener additive before but I'd like to add an aside.
The mix is 1 part water - 1 part vinegar. This solution best works on wallpaper that's actually made of paper AND glue that's acually 'homemade' as well. (This mix may work on other glues but it may take many applications.) If you have wallpaper that's not 'paper', you'll need a Paper Tiger to 'perforate' the wallpaper. Be sure NOT to press too hard with this tool as you'll 'ding' the drywall or whatever is underneath.
For the $$$, I think DIF by Zinsser is worth every penny. DO follow the instructions. (I suggest you read them twice so you're clear with them.)