Champion nerfs don't lead to core gameplay problems that lead to someone being put on a VGU list. Buffs or nerfs, as a major part of balance adjustments, will always exist no matter if the champion is outdated or fresh off the VGU shelf. Riot's point with the VGUs is to make the outdated champion kit useful again while adding more balance levers that would allow the champion to function properly and allow for nuanced balance changes that won't make or break the said champion. Binary champions (like Aatrox currently) are in this state where if you throw them a small buff or a nerf in any direction, they become super good in what they do or never see any play.
Riot did try to do VUs first and it turned out that full VGUs take much more time but are of a much higher quality and value instead of just updating the art and cosmetics of League's earliest champions and not changing a thing.
Furthermore (although i've pitched forward the same idea), 1 full year of no new champion releases and just full dedication to VGUs and VUs sounds enticing but that would mean a lot of content in League would get stale very quickly. Simply because we've gotten used to frequent changes over the years and we'd depend on VGUs to keep things interesting and change them up (bring something new to the table) while less intensive work like VUs would strictly be for eye candy purposes.
2-4 VGUs would mean pseudo-new champions being introduced while the rest being all VUs. That would be pretty much equivalent to Riot being more concerned about how champions look and not how they play. Both aesthetics and functionality is important however, if you have to choose one to focus on, it has to be functionality. Riot strictly chose to focus on VGUs first as they both change the functionality and aesthetics of a champion at the same time. Which is exactly why these are the top priority projects.