What OS are you running Tableau on ie Windows, macOS, Linux ?
What are the parameters set for the ODBC DSN being used which can be seen from the “odbc.ini” file (or registry on Windows) or provided via screenshot ?
What are the actual character being displayed incorrectly in Tableau, which you should be able to see from the correct data being returned when querying via the interactive SQL function in the Virtuoso conductor ?
Depending on the client OS being used we provide a sample ODBC application as detailed in the installation documentation ie cppdemo on Windows and iODBCDemo on macOS (found in /Applications/iODBC/ ) that can be used to make a test ODBC connection. If you make an ODBC connection with the same DSN and run the same query is the data returned correctly or with the indicated diamond chars seen in Tableau ?
What is the version, build date & gitid for the Virtuoso database server being used, which can be obtained by running virtuoso-t -? for open source and virtuoso-iodbc-t -? for commercial ?