Can not edit variants prices as a registered vendo

-- HikaShop version -- : 2.6.2 starter
-- Joomla version -- : 3.5.1
-- PHP version -- : 5.6.12
-- Browser(s) name and version -- : safari9.1
-- Error-message(debug-mod must be tuned on) -- : Can not edit variants prices as a registered vendor

Hi, I can not edit price variants as a registered vendor. I create 2 variants for a digital book, an epub as default variant and mobi as second option variant with different prices for viewing how the cart works. I see that the main variant price also put on the main product page automatically. I registered with the user name and pass for the test vendor I created. I choose the variants to edit but when I choose one variant, mobi for example it show me only the main product price.
I don,t know what I set up wrong. Also I attached a print screen. If you need the user name and pass for the vendor and the super user logins please let me know and tell me where to send you this info.

Regards,
Mihnea

OK. I find it. I changed the vendor user to author and all prices appear and can be modified. But now I have another question. The variant prices are with VAT. For example, the mobi files which I set it to 2 RON it appears at 2,4 RON. How can I setup hikamarket that vendors change the basic price, without VAT. VAT to be calculated automatically based on the price vendor change. For example, let's say that vendor wants to change price from 2 RON (2,4 RON with VAT) to 3 RON, and the system automatically put 3,6 with VAT after the vendor change the basic price?

Regards,
Mihnea