I used champagne vinegar but literally any kind will work.
The most common mistake with these is overwhipping the whites. Don't use an electric mixer for these.
If you wanted to dress this up by adding fresh chives, tarragon, or dill, that wouldn't be a bad idea. Or, you could do some kind of eggs Benedict presentation and serve on top of an English muffin with Canadian bacon--in which case you'd probably want to use lemon juice instead of vinegar in the egg whites.