So, after alot of reading, painful eyes and lotsa hair pulled out, here the path to tcpdf and fonts as of VM 3.6.2
/libraries/vendor/tecnickcom/tcpdf/fonts
the xml file has to be named with _specs. So, for example, you are installing dejavusans, which is what i have been trying to do, you need to name the xml file as dejavusans_specs.xml
_specs is called in /administrator/components/com_virtuemart/models/config.php Line 157, so your xml file needs _specs to be listed in Shop>Invoices&Emails>Appearance