No. Rare exceptions don't disprove the general rule outside of math. And also do not make the mistake of applying a value judgment to the word "ideology." Ideology is not a four letter word. Also, I generally dislike arguments that rely on "but not all," as when we have discussions like this, we're obviously talking about general rules, not every edge case. They always seem pedantic and gotcha to me.

Maybe you can argue vegetarianism is a dietary choice but not an ideology. Maybe you just don't like the taste of meat, or you just don't have much money for food. But veganism? That's almost always a deliberate ideological choice. It's primarily done so for ethical and moral reasons. You can be a vegetarian simply because you were born into a place like India. But veganism? That requires you to go out of your way to avoid animal products. You have to put the work in. And it goes way beyond simply dietary choices.

You can tell it's an ideology and not just a dietary choice because it's applied in areas far beyond simple matters of taste. Vegans will often carefully review food ingredients just to determine if there are any animal products in them. The product could taste absolutely nothing like meat, but for ideological reasons, they don't want to consume animal products. And veganism is often applied in areas well outside of food entirely. See vegan leather.

Or look up David Watson, the man who coined the term veganism. In his own words, he explained his motivations:

We can see quite plainly that our present civilisation is built on the exploitation of animals, just as past civilisations were built on the exploitation of slaves, and we believe the spiritual destiny of man is such that in time he will view with abhorrence the idea that men once fed on the products of animals' bodies.

That sounds an awful lot like an ideology to me. And I would argue that the vast majority of vegans do so for ideological reasons. It's not a matter of taste when you apply it to things you can't taste or even eat.

Or hell, you could just ask vegans. And people have done surveys. About 90% report being vegan out of concerns for animal welfare.

Veganism is absolutely an ideology. At this point it's pretty much a religion.