You can set the "<nop>TraceOn" param in the "[Parameters]" section of the  Virtuoso configuration file such that it is enabled on server startup  
as detailed at:

	[[http://docs.openlinksw.com/virtuoso/databaseadmsrv.html#ex_traceoniniopt]]


{{{
[Parameters]
....
TraceOn = soap, errors
...
}}}

This will enable logging of additional information regarding SOAP calls and SQL run-time errors into the virtuoso.log file.