The correct way to make a backlit print is to have 2 layers of color print with a white layer in between. That way you see 1 layer of color during the day, and 2 layers (when you need the extra saturation) when lit up. Color-white-color inkjet prints are the best way. Putting a layer of color on 1 side of white acrylic and another on the back is another way.
The term "backlit" usually means something particular as does the term "day / night."
From EFI VUTEk; "If the print is only going to be backlit, a three layer or white ink file is not necessary. In most instances, a simple file that is set to double strike, or creating a two-layer file with no white, will suffice."
The key here is that EFI is mimicking the legacy of previous photographic processes and also using a superior media much as before, polyester films, both translucent or clear options as desired. Also, the double strike mimics the longer exposure required by the previous photographic films.
Ideally, day / night does require two sided printing but the superior media is again the polyester translucent or clear, ink receptor on both sides. White ink is not necessary.
"Question about Clear Vinyl"
Polyester is dimensionally stable as is acrylic as is acrylic adhesive when used together. Vinyl, not so much. Polyester is rather stiff and may not need adhesive at all when sandwiched between a backer and face. An aqueous print usually resists sticking to a face. I'm guessing more backlits are produced using aqueous printers than solvent so far as items, but not overall square feet because grand format is in use as well.