The sales tax that VirtueMart charges a customer is based on your Virtuemart configuration when you into Joomla Administrator under top menu "Components -> Virtuemart". Then in left menu "Admin -> configuration", look for the section "Tax Configuration".

The table of configured the tax rates are listed in VirtueMart's left menu "Tax -> List Tax Rates".

More information about setting up taxes is in this section of the User Manual linked here.

How you want to setup your tax depends on your state and the advise of your tax accountant. Some general sales tax information here.