Pizza dough is often vegan when it contains just flour, yeast, sugar, and water.
Sometimes, other ingredients are added to the dough that make it not vegan though, so it's important to always check!
You always want to check before assuming the crust is vegan.
Ingredients to look out for: eggs, honey, l-cysteine, and milk powder or whey (sometimes found in "high-protein" flours).
Pillsbury makes pizza refrigerated pizza dough that's vegan. Many grocery store brands are also accidentally vegan.
I buy House of Pasta and GIANT brand pizza dough in the freezer sections.