User wants to add taxes to the service and expense line items on vendor bills to charge the clients with the same via invoices.
You cannot add taxes to the service and expense line items on vendor bills. But you can add them to the invoices that you create from the corresponding time and expense entries. When you create vendor bills, Core uses its service and expense line items to automatically create time and expense entries. You can then add taxes to these entries from the respective screens. You can also add main taxes on your projects. All of the taxes show up on the invoices you create from these entries. Check out this article for details: Taxes in Core.