VIGER VFX

Well, for one, auto-attacks are somewhat less important for VFX updates, since they don't encompass a space of "do I need to know how this works". They're short, simple, and all you really need them to communicate is "this person is attacking me".

For two...The VFX update is being done for Champions whose gameplay is fine, but just needs a visual facelift to bring to parity with other visual effects and bring more in line with Riot's standards for Gameplay - namely, having VFX match the actual hitboxes, having spells of varying import appear as important as they are, etc.

That's why they chose Champions like Veigar, VI, Jarvan IV, and Lee Sin - they're fine gameplay-wise, but some of their abilities are visually inaccurate on top of being old (Jarvan's Dragon Strike, Lee Sin's Safeguard, pretty much anything Veigar has to throw at you).

And that's the key point; they're not high priority gameplay update targets. Zilean, on the other hand, is fairly high on the list for a potential VGU, so it's understandable for them to want to avoid doing unnecessary polish work on something like Zilean's auto-attacks.

TL;DR, yes. Priorities. They have them fairly straight.