Sorry but serial IDs really should not ever be changed once assigned to a record if they, as appears to be the case here are used as primary keys to identify each record in the table and to link to other tables in relationships. Use a status field of some sort to distinguish between customers and vendors and keep the ID number unchanged.
Max, when used with this syntax returns the value of Vendor# + 1, the value you just set to zero in the previous script step. You'd need a self join relationship to get the value of Vendor# across all records in your table.
After a bit of trial and error, as well as someone helping me think it though, I have come up with a solution for this.
And if you have more than one user using this at a time, you can get two records with the same serial number value.