You can delete vendor invoices on your own. To do so, the status of the vendor invoices must be "Draft".
Status is "Paid"
If the status of the vendor invoice is "Paid", you first have to change the status to "Pending". To do so, delete the outgoing payment by clicking the "Trash can" icon in the "Outgoing payments" tab.
Status is "Outstanding"
If the status is "Pending", you can click the action "Edit vendor invoice again" so that its status switches back to "Draft".
Status is "Draft"
Then delete the vendor invoice by clicking the "Trash can" icon at the upper right.
Please note that the number range for the vendor invoices will not be reset.