Can I use wine instead of vinegar in salad dressing?

Yes you can, but the acid is a lot lower. I actually prefer to use citrus for the acid in most salad dressings. it's much more compatible with wine.

Agree with "Bigpan" - maybe bump the acidity with some citrus (lemon/lime/orange) depending on the recipe.

Yes, wine contains acid so therefore yes, but , since the acid is weaker than vinegar you will want to adjust you oil-acid ratio. Use less oil and taste.