I agree with Mike on this one, for a couple of reasons:
1) The whole point to vendor development is to ensure the value of incoming products and services. If their history and reputation are good, don't waste their time and yours on a time consuming audit. This is especially true if they are ISO registered (third party assessment means you don't have to).
2) Self-Assessments are only an exercise in creativity. Your supplier will obviously rate himself high, but not too high as to appear fabricated. They will not put any extra thought into it, and they will assume you won't either ("they're just doing it to pass an ISO requirement" the supplier assumes).
However, if the vendor begins to show signs of poor performance (e.g. late deliveries or non-conformities) then your Corrective Action Process should be used. If problems become chronic, THEN you may use a simple audit format (one where you actually perform an on site audit) to either help them improve or remove them from your list. This, IMO, is a last resort.
If you are choosing a vendor to use for the first time, and they are not ISO registered, then get information from their other customers first before determining if an audit is in order
IN ANY CASE, "Self-Assessments" do not seem to add any value at all.
Sorry for being so windy.