You completely nailed it. I'm very new to this field of study, but I just looked up dispersion and now I'm absolutely sure you're correct for both situations. Thin film interference seems to be the cause for the oiled wood, and dispersion for the vases. I feel stupid because I've completely failed to associate any phenomena with their proper terms (slaughtered it), but I think this topic is completely solved now. The vases do have an interesting gloss coating, so I think thin film interference is also playing a role. Thanks
By the way, what happens when the coating is thicker than allowed by the model in your article, and cos0 is very near to zero (don't think absorption, but dispersion)?