Dear friends, we had discovered a bug within Virtuemart which leads to improper tax calculation when VM default coupons are used or awo free versin is used. To fix this non-programatic way, you must set a specific tax rate per each payment and shipping methods, it MUST NOT be set to "Use default tax rate" as this will lead to tax malfunction. You also may use "Use no tax" per payment and shipping configuration.
The programatic fix is available here and hopefully will get implemented after VM 2.0.22+
http://www.rupostel.com/patch.1373554559.svn.txt
Best Regards,
Stan