Fallacy of "all or nothing"

Does there exist a fallacy for when somebody claims that because something isn't 100% true is must necessarily be false? 

EX: "Vegetarians say they're acting humane by not eating animals but they still wear products made by them."

Yes, the fallacy you're describing is known as the **"False Dichotomy"** or **"False Dilemma"** fallacy. This logical fallacy occurs when someone presents only two options or outcomes in a situation where more options are available. They present it as a strict either/or scenario, ignoring the possibility of a middle ground or other alternatives.

In the example you provided:

"Vegetarians say they're acting humane by not eating animals but they still wear products made by them."

This statement commits the False Dichotomy fallacy by implying that one must either adopt all possible humane practices or none at all. It ignores the possibility that individuals can make incremental changes towards reducing harm and that such efforts can still be valuable and meaningful, even if not perfect.

This example also may involve the perfectionist fallacy, which is a subtype of the False Dichotomy. The perfectionist fallacy occurs when one argues that a solution or action is not worth pursuing because it is not perfect. It suggests that unless a practice is 100% consistent or flawless, it is either futile or hypocritical. However, this overlooks the validity and positive impact of partially fulfilling ethical or humane practices.

In summary, the fallacy of "all or nothing" or False Dichotomy oversimplifies a complex issue by failing to acknowledge the spectrum of possibilities between doing everything perfectly and doing nothing at all.

Don’t overlook the human guilt factor. Remind them that every day, thousands of innocent plants are boiled to death or eaten alive and raw by vegetarians. I tell my vegetarian friends they should stop the violence — eat bacon! 

Of course, there are the technical fallacies mentioned in the response above.