How to store multiple vendor data in Google Cloud SQL?
The advantage of using a single database is in having a single schema and a single codebase.  If you make a change, then all vendors get the change.  (That can be a disadvantage, too, if they don't like the change).

Apart from that, the disadvantage is if the vendors have significantly different requirements and need different schemas and/or they interpret the data differently.  But if some of them want to have a schema or code change, then you have to manage all the different versions.

It's easy enough to serve up data for a given vendor to a given user by using the right queries.  Unless you want to be spending most of your time managing versions, you probably want to keep them all synchronized with the same schema and code.