I don't remember exactly which framework Sage 50 is running on but I don't think they have upgraded to 7 yet.

The SDK is not that sophisticated to allow you to use internal routines to update the different modules.  In order to update quantities you will have to create your own SQL statements.  I don't use the SDK so I can't help with that.

The table you need for the quantities is the tinvbyln table and update dHInStock or dInStock as needed as well as the appropriate cost fields.

For the Vendor linking, I am assuming the client is using Quantum as that is the only version with vendor specific inventory.  The main table here is tveninv.

Thanks for the reply, can you point me any docs for that SQL connection ?

SQL is not a connection.  You have been able to connect properly already because you said you have entered vendors and items.

SQL is Structured Query Language.  It is a language you use to communicate and manipulate the database.  The colloquial version you are looking for is MySQL syntax.  Your best bet for learning it is w3schools.com to start.  You can also find information on the mysql.com website.

To use the SQL commands you create will have to be done based on the SDK rules.  I assume you have read the manuals that came with the SDK in order to get as far as you have but if not, have a look at dv-progref.pdf and review the Record Updating and Transaction Processing sections and routines.  As I mentioned, I don't use the SDK so I can't help further with that.