I was wondering where I should add my own table update codes in VirtueMart, when a successful transaction occurs.
You could copy this file:
/components/com_virtuemart/views/cart/tmpl/order_done.php
Put your code and create an override to:
/templates/*template_name/html/com_virtuemart/cart/order_done.php
You could also create a plugin but plgVmOnConfirmedOrderStorePaymentData()
event is only for payment plugins.
Hope this helps