You can copy PHP files for view invoice to your template as override and next edit as you wish:
components/com_virtuemart/views/invoice/tmpl/invoice_history.php
components/com_virtuemart/views/invoice/tmpl/invoice_items.php
components/com_virtuemart/views/invoice/tmpl/invoice_order.php
components/com_virtuemart/views/invoice/tmpl/invoice.php
to
templates/your_template/html/com_virtuemart/invoice/invoice_history.php
templates/your_template/html/com_virtuemart/invoice/invoice_items.php
templates/your_template/html/com_virtuemart/invoice/invoice_order.php
templates/your_template/html/com_virtuemart/invoice/invoice.php
Or you can use some apps for invoice, example - Artio VM Invoice (is not our extension)