Hello,
Are you referring to using a default G/L account and dimensions when a vendor is used on a journal or purchase document? Or, do you mean the G/L account that the vendor "points to" via the posting group setup? What version of NAV are you using?
If you mean the former, you can set default dimension values on a vendor and/or G/L account which the system will insert on journal and document lines, but these may be overridden by other master data such as items or resources if they also have default dimensions and are inserted subsequently.
There is no out-of-the-box field for defining a default G/L on a vendor card but this could be done through development along with some custom code to auto-insert it when the vendor no. is used. You could also consider using Standard Vendor Purchase Codes, which are almost like vendor specific defaults but would still need to manually inserted on a line, and they're only available for purchase documents, not journals.
Regards,
Rob