Preheat oven to 400°. In a large bowl, toss cauliflower with oil, vinegar, garlic powder, and cayenne. Season with salt and pepper and spread in an even layer on a large baking sheet. Roast until florets are tender and slightly golden, 20 to 30 minutes, depending on the size of your florets.
When ready to assemble pizza, return cauliflower to large bowl and toss with 1/3 cup barbecue sauce. Turn oven up to 475°. Grease a large baking sheet with cooking spray.
Make vegan ranch: In a medium bowl, mix together all ingredients then season with salt and pepper.
Spread pizza dough onto greased baking sheet. Spread remaining 1/3 cup barbecue sauce on crust in a thin, even layer, leaving about a 1/2” border. Top with cauliflower and red onions, and bake until crust is crispy, 12 to 15 minutes.
Garnish with chives and a drizzle of vegan ranch.