ok, a little bit delay but is resolved.
When order status updated from the backend or pdf invoice saved in safepath, controller runs a function that set the fonts again. So edit the file joomla_folder\components\com_virtuemart\controller\invoice.php
around lines 151, 152 change the font
$pdf->setHeaderFont(Array('freesans', '', 8 ));
$pdf->setFooterFont(Array('freesans', '', 10 ));
also around line 179, change also the font
$pdf->SetFont('freesans', '', 8, '', true);
Sorry for taking to long to respond but i was thinking that this was working, find it today while upgrading a live site. So each time you upgrade vm2 you have to change also that lines!
Regards