I love Vinaigrette Dressing and it is a lot of fun to play with its different variations, as there are so many types of vinegar and oils in the market nowadays. The traditional ratio for a vinaigrette dressing is 1 part vinegar for 3 parts oil. However this will depend a lot on personal preference and on the type of vinegar you are using. Some vinegars are stronger than others and you might want to add more oil if the dressing turns out too acidic.
Another way to cut the acidity is to add something sweet like honey, maple syrup, fruit juice, fruit preserves or pureed fruit. I would avoid sugar and sweeteners.
For a summer sald vinaigrette you can use lemon juice instead of vinegar but usually the best is to use lemon juice to enhance the flavor of the vinegar.
As we all know oil and vinegar do not blend together indefinitely. You shake them and later they will separate. Mustard and honey help to emulsify a vinaigrette, meaning if you add any of those two ingredients to a vinaigrette the oil and vinegar won't separate.