If I might add a question to this debate:

If most of MLV sheet's surface is secured to a vertical panel (like a plastic trim card on a side door) using glue or plenty of velcro strips, I don't see a need to use any decoupler between the MLV and the trim card.
The only way for MLV to vibrate in that setup is if the entire trim card would vibrate with it, and that would take a lot of force to "excite".

Is this thinking correct?

By the way, has anyone used an automotive-grade glue (example: roof upholstery glue) instead of velcro strips to secure an MLV sheet to a vertical surface? Did it hold up in the long term?