I know that VirtueMart cannot do partial refunds with PayPal payment plugin and they must be processed from the PayPal Merchant account according to this advice http://forum.virtuemart.net/index.php?topic=134495.msg467598#msg467598 (http://forum.virtuemart.net/index.php?topic=134495.msg467598#msg467598).
This is a common function required by online shops so I'm appealing to the developers to include this in the next major release so that my clients can handle this from within VirtueMart instead.
Thanks in advance!