i am not sure exactly what your data encompasses. you need a VAT indicator to give with each line - now this could be either
a) an actual value or
b) a vat percent rate or
c) a numeric indicator identifying a vat rate in another table
and you possibly also need a VAT indicator flag, to differentiate standard adn zero rated items
all of these have different pros and cons
also given that there are often rounding differences - eg, a suppliers VAT calculation may be different from yours then it may be necessary to store both the VAT amount AND the VAt Rate.