Vercel is understandably trying to shift all the blame on the third party but the fact their admin panel can be accessed with gmail/drive/whatever oauth scopes is irresponsible.
If you can only fix one thing (ideally you'd do both, but working in infosec has taught me that you can usually do one thing at most before the breach urgency political capital evaporates), fix the Google token scope/expiry, or fix the environment variable storage system.
IMO it's probably a bad idea to have an LLM/agent managing your email inbox. Even if it's readonly and the LLM behaves perfectly, supply chain attacks have an especially large blast radius (even more so if it's your work email).