Dear,
Item category determination is depends upon VOV4 settings only to get into VA01 .Just try one scenario as maintain VOV4 settings as OR with TAN and maintain Copying control with differently in VTAA then findout what will be happened .Actuall,it should get VOV4 settings only .But coming to the point is If you maintain settings like OR with TAN and TANN ( default item category then ,If you maintain thsi TANN in VTAA then system will take the TANN only not TAN ). Just it and let me know .
Secondly is ,VA41 ,you maintained 0001 customer then you are creating this VA01 with VA41 ,now here system assume you as thses both VA41 and VA01 should have to same customer .
Test as maintain different customer in VA01 and let me know the result means maintain that routine in VTAA and create VA41 with 0001 and try sales order with different customer then let me know the result .
Thanks,
Naren