It depends. At Subway sandwich shops (the fast food chain), "oil and vinegar" means a blended vegetable oil and an inexpensive red wine vinegar. That is all. Some Subway shops may have one oil bottle and one vinegar bottle and put some of both on when you ask for oil and vinegar. A better Subway will have a third bottle that is a mix of oil and vinegar that can be shaken before applying to make it temporarily suspended, which also ensures a consistent proportion of oil and vinegar.
I find that at Subway, the oil and vinegar topping really works best if you also have the "Italian seasoning/herbs" and "salt and pepper" added as well. The latter is just salt and pepper. The former is actually Italian herbs including oregano, thyme, I think rosemary, and maybe some basil. The oregano and thyme are definitely prominent.
Other delis and sandwich shops are probably doing something similar to Subway. They may be mixing different proportions, using better or cheaper oils and vinegars, and/or they may mix the herbs and salt and pepper in with the oil and vinegar to make a complete dressing.
If I were doing this at home, I would make my own blend inspired by my favorite sandwich shop. I would use:
- 100% extra virgin olive oil instead of an oil blend, but if you find the oil to be too strongly flavored you can switch this for a canola oil or soybean or blended oil. Nut oils like peanut might give a strange flavor.
- A nice red wine vinegar and not balsamic, because balsamic could be too strong and wasted on a sub that already has strong meat and cheese flavors. I'd go with about 2/3 oil to 1/3 vinegar and adjust to taste. You could try a white wine vinegar, malt vinegar, or cider vinegar but the latter two will taste much less "Italian".
- Salt and finely ground pepper to taste.
- Herbs, strong on the oregano and thyme, I might put in dried basil also. Rosemary would have to be finely ground or omitted because it can be "spiney" if not cooked in.
I would put it in a bottle I can shake vigorously right before I apply it, and I would want to apply it to a pile of shredded lettuce and other vegetable so it doesn't just run out, and I Would want the bun to soak it up a bit.
Instead of shaking vigorously, you could add a little bit of mustard flour or prepared mustard or an emulsifier and whisking it or shaking it to emulsify it before applying. Again, this helps ensure it applies evenly and the proportions stay constant.