You may find it helpful in attempting to change the language in your shop from 'vendor' to 'third party' to get others thinking more broadly. While I tend to often be at loggerheads with this over the lead of our 'Vendor' management committee (he heads up IT and deals with 'vendors'), it gets other department personal think a little further about their relationships, and ends up improving their buy in in completing (or at least attempting) required due diligence.
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I don't repeat gossip, so listen closely...