Hello,
The setting of your user points instance configuration will very interesting too.
Since it's the user points which makes you think that there is a wrong VAT total ; I suspect that the discount generated by the plugin is not set-up to reduce the taxes.
Regards,
Jerome - Obsidev.com
HikaMarket & HikaSerial developer / HikaShop core dev team.
Also helping the HikaShop support team when having some time or couldn't sleep.
By the way, do not send me private message, use the "contact us" form instead.