Let's stop pretending she's an assassin. She's a tanky fighter - champs like her usually don't have reliable escape and their abilities aren't as reliable as unstoppable, targeted knockup that follows you nearly forever.
Fighters that don't have escape abilities after they used their initation tool (some don't have any even if they don't use the initation tool): Aatrox, Darius, Garen, Gnar, Hecarim, Irelia, Jax, Nasus, Olaf, Poppy, Shyvana, Sion, Skarner, Trundle, Udyr, Warwick, Xin Zhao, Yorick, Maokai, Malphite, Amumu, Rammus, Sejuani, Shen, Zac.
This is not a problem for tanky fighters such as herself, since she is supposed to stay in the fight, not just assassinate and run away. Also, you are saying that if carry is in 1525 range of Vi then he's out of position (that's Q and R there, of course Vi can also use flash, but adc can do that too, so I will ignore it)? And once you are in that range - there is very little you can do, except getting hit with it and praying that her team will not follow immidiately, giving you chance of using your escape spell (be it flash or champion skill) after the knockup ends. And no, you cannot remove knockup in any way possible (QSS, Mikael and Cleanse - none of it removes it).
Comparing it to other similar spells clearly makes it the most universal choice, also just being mostly superior to Jarvan ultimate. Malphite have more range and bigger AoE, but it's a skillshot and can be dodged (usually with a use of summoner spell or champion spell, as malphite is rather fast during his ultimate). Jarvan on the other hand have less range, hits only 1 person and doesn't chase you. That means if you use flash before he hits you - you will most likely end up outside of the walls (you still take damage though). Also, even though it is strictly a single target spell it only deals +10% bonus AD more damage than Vi's ultimate. And Jarvan ultimate doesn't apply any CC whatsoever.
Of course, there are situations where Malphite or even Jarvan ultimate will suit the job better - there is no denying that. But Vi's ultimate is going to be the best choice in most of situations, while very rarely being the worst choice.
As it is right now, Vi's ultimate provides very little counterplay to enemy team. IMO it should either have it's range nerfed (as stated before, with compensation to something like Q range) or give it a slight channel time before she starts chasing, in which she can be interrupted (let's say 0,75-1s).
...
Holy shit, you can add champ portraits on boards. 
