it does still retain some reflectivity. We had a heat transfer printable reflective material at one time that worked well and I have printed onto reflective decal media many times. the darker the color, the less reflective.
I print all the time on 3M 680C reflectives... the tranparency of the CMY colours allows the reflectivity of the media to still shine through, but the K is nearly opaque, so anywhere printed in black will not reflect anymore.
when you print on reflective vinyl do you then laminate it to protect it. I usually print on a clear material, laminate that then stick it onto the reflective, I find it works quite well and doesnt fade over time