No, it will calculate what portion of the $100 gross price paid would constitute that 20% VAT -- i.e.:
price - (price / (1 + VAT rate)) = VAT paid
...e.g., for 20% VAT on a $100 price:
$100 gross - ($100 / 1.20) net = $100 gross - $83.33 net = $16.67 VAT