We've based it upon this line out of
Types, Subtypes, and Special Abilities
Quote:
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A few other creatures require combinations of different types of attacks to overcome their damage reduction. A weapon must be both types to overcome this damage reduction. A weapon that is only one type is still subject to damage reduction.
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So for the vamp, weapons have to be both silver and magic to bypass their DR. The MM (yes, I know its not OGC) does have specific examples in it to help.