I think OP has mixed two questions into one. First being confusion with the ChatSession he/she must have come across in the SDK docs, which they probably think it’s somewhat going off their device and Google doing the storing of the history.
Second is using one google account to have multiple users interact with their vertex ai model. This should be straight-forward if he/she is able to achieve the same with Gemini API. I don’t see why second scenario should present any challenges because currently there’s no way to have conversation history stored (haven’t checked availability of Vertex Ai context caching).