Hello,
This is probably not translatable at the moment.
You can modify code in: administrator/components/com_vminvoice/helpers/invoicehtml.php - around line 95 is:
$replacement = 'no shipping date';
Modify it to:
$replacement = $this->_('COM_VMINVOICE_NO_SHIPPING_DATE');
Then you will translate it in ini file (frontend language files):
COM_VMINVOICE_NO_SHIPPING_DATE="no shipping date"
Translation for Variable Symbol I found only in admin lang files. There is not this string in code.
Try add there this translation (in admin language file):
COM_VMINVOICE_VARIABLE_SYMBOL="Variable Symbol"
First missing text I will immediately modify, so it should be in next release.