Hello Daniel,
if i correctly understand your request, you just have to maintain locally your CAT attribute value.
For organizational unit DEPT A, its CAT value = VENDOR1 cat and you delete this CAT value from the company Org. Unit.
Check also what is the configuration for this attribute in OOATTRCUST transaction: the attribute/scenario should be "Local values overwrite inhertited values" or "No inheritance".
Regards.
Laurent.