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I don't know why, but most folks goes with a blend material for such simple task like decals. While blend is extremely useful material in certain scenarios, it also takes noticeably longer to render. In many situations, single material with composite map is more than enough.
As for decals itself, usually you'd want to control colour and opacity with separate texmaps. That way you'll gain most control over your material. Try to google for vray decal tutorial, i'm sure there's plenty material about that on the web.
