To answer your question - lots of creamy salad dressings have vinegar in them - it won't hurt the buttermilk.
I have a GREAT low fat potato salad recipe made with FF Sour Cream. Guests that I've served it to didn't even realize it wasn't the regular full-fat mayonnaise dressing:
Denise’s Low Fat Potato Salad
3-4 lbs. potatoes, peeled, cooked and cubed
1.5 c. thinly sliced celery
1 bell pepper - chopped
1 med. chopped red onion
optional - 2 hard cooked eggs - chopped
Dressing
1 c. Fat free sour cream - I like Naturly Yours brand
1/3 c. low-fat Ranch Dressing (I like Wishbone, Just 2 Good brand)
1/3 c. sweet pickle relish
1 t. celery seed
1 t. dill
1 T. vinegar
1 T. prepared mustard
2 t. salt
1 t. sugar or 1 envelope sugar substitute
1. Mix dressing and allow flavors to blend while preparing other ingredients.
2. Peel and cook potatoes until tender, but not overly soft. Plunge potatoes into an ice bath to cool. (Eggs can be hard cooked with the potatoes if desired.)
3. While potatoes are cooking, chop celery, bell pepper, and onion.
4. Cube cooled potatoes, chop eggs, and fold dressing and all ingredients together. Chill at least 4 hours, or over night for best flavor.
It may look like too much dressing at first, but the potatoes will absorb the extra dressing as it chills. If you don’t have enough dressing, stir in a little more low-fat ranch dressing.