It will not be easy but you can try following way. 

Setup>Users>Row level permissions enter a  new row:

- Object=BPS

- permission code create new code or chose one from listed - let say BPR

- field eneter BSGCOD (BPS category)

After - Setup>User>User>Organization[Tab]  in Row level permissions grid choose permission code BPR and specify in key field your BPS categories to which you want grant access. 

Test it - probably it will deny access to other categories - clients as well - then you will need to add additional conditions for them. 

Thanks Grits. Could you please tell me how can I add additional conditions and where, that you suggested in the answer.

Have you already decided how you will handle case when the same vendor is "Purchase Module vendor" and "AP module vendor"?

There is no case such Purchase Vendor in AP Module and vice versa.

May I know why you asked a valid case which can be come up in future time

Ok - got a bit more time for test solution as well.

What I did - created new Permission code [BPS] and entered into [Field] BSGCOD - which points to BPS category.

For my test - as supplier are divided by countries - I choose BE supplier category(Belgium suppliers). 

After -  edited user settings (module SETUP) and in Organization tab Row level grid -  entered permission code BPS and in key field specified BE as criteria. 

As result - user was able enter invoices only from Belgium suppliers - but independent from invoice type (Purchase invoice and BP invoice).

Ah - and it doesn't restrict any other BP category - only BPS.