Picking viktor into leblanc in any draft scenario is a bad decision. Of course this was teambuilder, but just remember leblanc makes mincemeat of non-mobile mages. I would say this matchup is possibly more severe for viktor than Ahri is.

The main thing is that depending on teamcomps Even I would would sometimes pick viktor into leblanc if I have good frontline composition. The later the game goes the more useless she will become. Also to really shut down a leblanc all your team needs is at least two tanks on your team and to group religiously. Its like the opposite of katarina. The more cc you have and tanks I have to jump past the more impossible it is for me to ever go in on your team. Clumping is a great strategy against her. She doesnt have safe waveclear so if you can just get your team to siege mid she has to give it up unless her team groups too, and if they do shes leblanc. She doesnt really help teamfights besides damaging one target and then becoming useless (unless of course shes fed as we all have seen at some point)

Also I would probably either rush a spectres cowl and then my standard build or just straight up rush an abyssal scepter. Once you have enough MR even if she dives you can survive the burst. Now if shes ahead and snowballing even the MR wont mean much.

Viktor definitely has enough waveclear and Aoe that he can just survive laning phase and get a good amount of farm. If you focus on not trying to fight her early you will just outscale her and be more useful. Not having any MR at all against a Leblanc is really bad news. Shes literally only can deal Magic damage so once the item is completed it pays off so much against her.