If it were stored in a cool pantry--perhaps in a basement, it would likely remain optimal. Vinegar doesn't go bad.

So, what's left? Well, dried herbs would probably not be a major issue for bacterial contamination, nor fresh herbs that you wash well. If sugar is added to the dressing, it would potentially have some bacterial-inhibiting (bacteriocidal) effects. There are some reasonably rare issues that could come up, but are mostly an issue if very young children (infants) were to eat your dressing (e.g., botulinum spores in honey or potentially on dried herbs from questionable sources). But, I wouldn't think the risk would be at all likely for your dressing--I just feel the need to mention.

So, leaving it out in hot temps. Bad idea. Storing it out of the refrigerator in cool pantry-type temps (e.g, 60 degrees or cooler)--probably pretty optimal. Mustard should not be an issue. Any product with egg or mayo WOULD BE and must be refrigerated.

Oil congeals under refrigeration, but it also takes way longer to go rancid (my mother has kept canola oil in the fridge for years, mostly because it takes her a year to use a full bottle!).

There is nothing that typically goes in vinaigrettes that should spoil. We all keep oil and vinegar at room temp, and a lot of us keep mustard out, too (if you wanted to get fancy with your dressings). Keeping out leftover vinaigrette for a day or two shouldn’t pose a risk.

That said, two other options to present: make less! With a little practice, you can figure out exactly how much you need to make for the number of diners. And if you get a sealable bottle (I use old spice bottle, like McCormick, because it’s just my wife and me), you can wack in all your ingredients and give it a shake to emulsify and bob’s your uncle your got dressing.

Option 2, if you’re nervous, is just toss it. I generally hate to waste, but unless you’re making a cup at a time the cost of ingredients used is pretty minimal (but even better if use option 1!).

Now, all that said, one caveat: I live most of the year in a country that leaves mayonnaise and eggs on the counter, so YMMV.

Good luck!

I got a second small refrigerator to keep at 60 degrees in case the house goes over my tolerance. I can handle up to 84 degrees, which is not so good for food.

It might be silly, but I bought some bottles on Amazon (told you I'm a novice). They have screw-on caps and recipes written on the sides with lines to show how far to fill it with this or that. I've only tried the Italian because it didn't work out so well. Then I tried online recipes. I think I could maybe mix up the herbs in a batch and use less of the oil and vinegar and then just introduce the herb mix as needed. My house has air conditioning but it sure isn't set at 60, lol.

I've lost a lot of weight in my effort to be healthy and I've learned a lot along the way. I do still get fuddled.

I'm glad to know it was the oil! My inner mad scientist is very pleased to know I was right about that.

And vinegar, it’s probably fine in room temp. But, I think salad dressing holds together better if it’s cold.
I make oil, balsamic, and Dijon mustard dressing a lot. And it’s good room temp. But, it’s better cold.

I would leave in fridge overnight and out during day as a compromise.
Personally when I have refrigerated vinaigrette with olive oil and forget to take it out early enough I just run lukewarm water over the jar or set it in a bowl with same.

Congratulations on shedding excess weight!

Consider trying yogurt salad dressing -- https://www.google.com/search?q=yogurt+salad+dressing&udm=14

This Turkish dip is so similar to Green Goddess dressing, you can use the recipe unchanged - https://www.tasteatlas.com/cacik/recipe (Huge recipe, try 1/3 scale !)

… and simply cut the recipe by that much? No leftovers, no worries!

it’s there to thicken the oil/acid emulsion, but I’ve never noticed that it had much effect. Or noticeable taste.

I make my own salad dressings, usually the good California olive oil from the farmers market, whatever vinegar catches my fancy, and various dried herbs. I use a small jar that once held curry paste, and the ingredients, cover and shake well. If there’s any leftover, I’ll refrigerate it if it contains eggs, anchovies, or dairy, definitely, and may anyway.

Not a fan - even a little bit!

I haven't gone to the store and picked up new things yet but I tried a balsamic vinegar dressing today. Holy cow was it strong! I used the scary mustard - though less than the recipe called for and didn't even notice it. Could be because the vinegar was so strong. I'll try with more next time. I normally use too much dressing on my salads. This could help as it was so potent. Used so much kitchen equipment to make it I had to run the dishwasher!

Put it in the fridge after. If it congeals, I'll try getting it out early and/or the warm water trick.

I feel like I'm making a little progress here.