When we invoice somebody, after completion of the invoice it should prompt that it should be billed to some vendor too. And if somebody skips the entry it should flash a message "are you sure you dont want this product to be billed to the vendor" The prompt will reduce the chance of missing the entry. and the books could be maintained diligently. This will be helpful for the service industry where many a products are being bought and sold the same day.
Mario Rodriguez commented
Completely agree with this. In the ideal case when we create a Sales order or Invoice we could also associate that invoice with a vendor or vendors. Right now when we create a sales order or invoice we can only add the customer information not the vendor information, the problem we are facing with this is that, in Sales orders or Invoices, Vendors are not connected in the process. Productos or Services are not connected to Vendors either so at the end of the day the only way we have to connect Sales Orders to Vendors is by adding the Vendor Data as "Sales Person" in the Invoice and then in that way we can create a Report in Zoho Books where we choose the Report "Sales by Sales Person" and in that way we know which invoice belongs to which vendor. This is being our biggest issue in Zoho Books for us and we do not know if Zoho Books Developers will ever consider adding this feature: Adding Vendor information to an Invoice.