yes, vminvoice 2.0.27, joomla 2.5.14, vm 2.0.22
in VM there is all ok, send me screen of your config of Invoice Items Configuration. On my screen the line coupon has no minus before value. In yours screen there is, we use % discount coupons...
if i choose option Show Total Tax Line - without coupon discount, system shows on invoice Vat from items, if i choose yes - deduct coupon discount - value of vat is vat from items + vat form discount, right should be vat - vat discount, maybe because of missing minus before discount value...