Vinyl decals should be tested with MirrorCoat by the user to ensure it meets their satisfaction. There are two potential issues that can arise.
First is an uneven distribution of the MirrorCoat if the vinyl is not level with the substrate. This depends entirely on the thickness of the decal. You may be required to do multiple coats over the decal until the MirrorCoat is level with the substrate.
Second, there is some concern with de-wetting. The Mirror Coat will need to overcome the surface tension of the vinyl for it to adhere well to the substrate. You can easily test for this by doing a seal coat over the decal. If it works, then great. If not, then you'll want to see if the mass of the flood coat (4 oz. per sq. ft.) will overcome the surface tension. In general, it does, but you will still want to do a test.
Pending the two tests above, the user may opt to pre-seal the wood entirely, lightly sand, and stick the vinyl decal over the pre-sealed surface, followed by the flood coat per our instructions in the MirrorCoat Brochure.