In a mini food processor or blender, combine the parsley, chives, anchovies, garlic, tarragon and capers, if desired, and process to a fine paste (or grind with a mortar and pestle to make a fairly smooth paste). Blend in the mayonnaise, sour cream, and vinegar. Scrape the dressing into a bowl and season with salt and pepper, to taste. Refrigerate until serving time. (To make ahead: The dressing can be refrigerated for up to 3 days.) Makes about 1-3/4 to 2 cups.