Override or change Tax rate when Tax Exmeption Number have entered

[chong]

How can we change tax rate to 0 if the cart user have entered Tax Exemption Number on the Billing information?
I have tax rule "CT State Tax" with 6.35% tax for Connecticut only. If the billing information have tax exemption number, I want to change it to another tax rule "Tax Exempt" with 0% tax.

Joomla version : 2.5.16
Virtuemart version : 2.0.24

Really need help for this one,
Thanks,