Natural butter flavor isn’t the most common ingredients, but I have seen it in some foods and recipes.
Considering it’s supposed to be used when butter isn’t an option, that suggests that it might be vegan.
While it depends on the brand, most natural butter flavor seems to contain:
Alcohol, water & natural flavors
For example, this butter extract on Amazon is made of “Water, propylene glycol (alcohol) and natural flavor.”
Alcohol (not that kind of alcohol) and water are both vegan of course, but natural flavor is a question mark.
The term “natural flavors” is an umbrella term for flavoring ingredients derived from plants and animals. Some natural flavors are vegan, but some aren’t.
It Depends on the Brand
I suspect that most natural butter flavors are vegan, but some probably aren’t.
If you’re buying some, look for a brand like The Spice House that specifically states that their product is vegan friendly.
Otherwise, contact the manufacturer of whatever food you’re buying and ask if it’s vegan or not.
It’s a pain, but there’s no other way.