I'm not sure what you mean. VertexAI is a product in the larger Google Cloud portfolio. It makes sense that they house everything together instead of making disparate platforms for each. This makes authnz consistent for me and simplifies their end too.
In addition to the models, you'll find a host of day-2 features like model monitoring and experiment tracking. Having to vet and pick from 100+ new SaaS's for these is a nice to not have problem.