Hackaton, from Dave:

... conversation centered around the access patterns between Google AI Studio and Google Vertex AI.  Your application currently uses Google AI Studio API keys.  Even though Google Vertex AI supports a number of different options for programmatic access, managed API keys would be the most similar. See also key REST header. Please see Authentication methods at Google for more information.

Best practices would encourage you to restrict your keys as much as possible.  The API key method of invocation would charge usage to the billing account associated with the project and key, but end users could pay for their own usage by setting up their own billing/project/key environment.