Which VGU will come first?

Yeah, like Asamas said. Swain's production began when Urgot's VGU went live, and now that Evelynn is live, the team that worked on her is switching over to Irelia. We should see Swain's rework possibly near the end of this year or closer to the beginning of 2018 because the team has been working on him since the end of July, and Irelia should follow closer to April or May of next year if they maintain their 5-6 month pacing.

It most likely to be new champ -> Swain -> new champ -> Irelia and sometime in beteween Aatrox or first Swain ->new champ ->Irelia -> new champ. That are the two options. But yeah Swain will be before Irelia.

For VGUs, it's always the one that you knew of first (Swain) who will be the one that comes first.

Their set up will generally look like this though with all the info that we have so far

New Champ (bubblessparklessleep) > Swain (VGU) > Irelia (VGU) > Aatrox (GU, last cause they don't have a clear direction as they've said)

https://support.riotgames.com/hc/en-us/articles/202294884-Champion-Update-Schedule

Swain, Irelia, then Aaotrox. Aaotrox will be the largest gameplay update to date so he might be pushed back again.