Hello,
it will be solved in the next release by adding configuration possibility of "tax tolerance match".
That is that e.g if calculated tax would be 18.79%, but there is defined 19% tax in your VirtueMart, you need to set tolerance to e.g. 0.25% so that the result 18.79% is matched with th 19%.
Unfortunatelly, VirtueMart data structure is really stupid in this - meaning that it does not store all the values, but some of them need to be backwards calculated - which with small prices causes rounding problems.