Here's the thing; all of a champions skins will be updated with a VU, including legendary ones. The caveat here is that they will be updated to be the modern standard of the skin itself, not the pricing tier. This is why Firefighter Tristana kept her water vfx and some special sfx, but didn't receive any new animations or a new VO. They brought the skin to the modern standard but did not give anything more than what was already included with the skin.
That's how Riot's approaching legendary skins and a champion having legendary skins does not have any impact on whether or not they'll get a VU. Chances are if the skin is new enough and of a high enough quality (SG Udyr and OS Teemo for example), then those skins will probably be left alone when those champions are given their updates.