Customizing tax in virtue-mart is not an easy task to do.
1.go to ps_checkout.php in classes folder
2.custom the code in there about the task you want to.
3.you have to be carefull in doing customizing the code cause it affects all virtue-mart process.
4.Just touch the code that ralate to the issue.
5. create your own function so that will not affect the other functions in there
6. You can use the other function through your fnction created.
