I've used GCP Vertex AI for a specific task and the prompt was to generate a JSON response with keys specified and it does generate the result as JSON with said keys.
Issue is that's it's not guaranteed, unlike this new openai feature. Personally, Ive found Vertex AI's json output to be not so great, it often uses single quotes in my experience. But maybe you have figured out the right prompts? I'd be interested what you use if so.