Hi,
The problem comes from that plugin and how it was coded.
I can see that it bases itself on a custom plugin Jerome did at one point to assign a vendor based on the total amount of the order. That is correct.
However, the trigger onBeforeProductsVendorAttribution used in that plugin is not meant to be used to set the prices of the products.
That's why it doesn't work as expected.
Instead, the code to change the prices/taxes should be in a HikaShop trigger like onAfterCartProductsLoad, as a HikaShop plugin (so another plugin).