Regarding questionaries not being filtered by vendor.

I have a question regarding the distribution of Questionnaires to vendors through the Procurement category:
When I add a Questionnaire to a specific node in the hierarchy, where I also have specific vendors assigned, and in this very node I change the Questionnaire Activity type into “vendor management”, this Questionnaire is immediately published in the Vendor portal for those vendors.
So, this is what we want, but our problem is that the Questionnaire is also published into the Vendor portal for all other vendors (even if they are not assigned to the Procurement category in any node). This will lead to not relevant questions for the other vendors etc.
I have also tried to limit the access to the specific Questionnaire, through the Questionnaire group feature (where I add contact persons as group members), but it still doesn’t add any restrictions into the Vendor portal: All vendors still have access to all the Questionnaires which have been assigned to the Procurement hierarchy.