Let me jump in here and say "yes!" it's fine to have regular dressing or oil & vinegar. The alli plan is really quite flexible: you can follow the Sample Menus or make your own. What's important is to have the recommended amount of calories and fat for your level + a variety of foods.
You can "spend" your fat grams however you like. Some people prefer a little more chicken or some butter on their baked potato and choose FF dressing; other do the opposite. It's a question of preferences and trade-offs.
Please let me know if you need anything else.
Pat Baird, MA, RD, registered dietitian