I want to change the sort order in a pre-defined language. How?

For example, changing the sort order for tone marks in Vietnamese to GRAVE, HOOK ABOVE, TILDE, ACUTE, and DOT BELOW:

SQL>CREATE COLLATION new_vietnamese_3 FROM vietnamese_3
SQL>USING 'Ĭ#<<#0309#<<#0303#<<#0301#<<#0323#';