You can't justify an ability my naming a single champion that counters it. The balancing point for Vi's ult is not "Tahm Kench."
The problem is that Vi's numbers are fine and she is out of the meta. However, even so, I do believe it's such an overloaded, toxic ability that packs too much guaranteed punch into one spell. CC, mobility, Anti-CC, potential for greater CC, damage, and unavoidable point-click lock on. I typically am against just listing all the pros of a kit or ability because it inflates it in words past what it really is, but there's no way to shorten Vi's ult.
She knocks you up, knock anyone in her way back, does decent damage, click lock-on target, is immune to all CC throughout the flight, and is essentially a slow, but unavoidable (Besides mages and Tahm) gap closer.
Even in practical sense, you can take away the knocking people back. It's still knock up, point click mobility, damage, CC immunity. The counterplay is that it is an all in and Vi has very little way of getting out, so the idea is she's fully committed but I think that is a poor trade off for what you get from it. If it was a skillshot, I think it would be amazing and open it up to enough counterplay that they could even afford to buff its effects a bit, but as a point-click, it's rather silly. It's the only point-click spell in the game that is just so jam packed with effects. The only comparable point-clicks are Veigar's ult, which quite literally only does a planetary fuckload of damage, or maybe Zed's ult that involves two potential forms of mobility and a brief window of untargetability along with damage.
Although Vi is completely balanced and not overbearing, I do believe her ult, by design, is flawed and unhealthy for the game. It's like Yorick. Fairly shit during mid/late game, but he is completely toxic and about unbeatable/unfun in lane. Considered the weakest league champ, but still no one has fun with him in game. Vi's ult mostly gets talked about when she's highly in meta. Until then, we won't hear anything about it really.