Vendor Approval

I have learned that approving a vendor for a particular part class tends to
unapproved purchase of that class from anyone else who is not specifically
approved for that class, where if no one is approved for a part class, we
can purchase it from anyone.
I hope that part was clear, because...we are looking at approving Vendors
for particular Customers, and I am wondering if the same logic will apply.
That is when I approve Vendor 123 for Customer ABC does that mean that I
will not be able to buy material for Customer ABC from anyone else unless I
specifically approve the other Vendor for Customer ABC as well.
What I would like is to have this field limit purchases from Vendor 123 to
just material for Customer ABC and nothing else. Yet, I would also like to
be able to purchase additional material for Customer ABC from other vendors
as well.
Can anyone tell me how the logic does work, and if Vantage will do what I am
looking for using this field?
Vantage 5.0 is our current version.

Thanks,
Aaron Hoyt
Design Standards