To clarify, is this only one user who needs to be looking at all stores but only two vendors, but when focusing on vendors he/she can see them all but only two stores? Or is it two different users, one of which can see all stores but only two vendors; the other can see all vendors but only two stores?

If it's the latter, then go with what wildh posted, create two roles (one for each user type) and within role1, set the vendor dimension to a limited subset; and in role2 set the vendor dimension to a limited subset.  Remember to put each user in only one role.

If it's the former, i'm honestly not sure how you'd do it with roles.  One alternate method would be to combine the vendor and store dims (if possible) so that there is a relationship between the two.  But for more than one user this would mean creating dims specifically per user type