Blowing the enemy Flash is only useful is you capitalize on it, and I do not consider my ganks successful if I that's all I get out of them. Obviously it is a good thing, but it often requires the jungler to pay attention to that lane while the cooldowns are up. There are so many strong jungle gankers. Are you guys really going to sit here and tell me that Vi ganks measure up to what Maokai, Nautilus, Rammus, Lee, Sejuani, Nocturne, Fiddle, and Jarvan can do? I could keep going too is the sad part. Vi ganks are pitiful, and I'm not afraid of her with or without Frozen Mallet (which realistically is not happening early in the game).
But Phage (or Red Buff) would be relatively early. I merely mentioned FM since it's notable for late game escaping her, not due to expecting it early game.
And yes, I'm going to sit here and say that her gank measures up to some of the best jungler ganks. Because she does. And she's considered a strong ganker once she has her ult because of it. It's one of the best ganking ults in the game.
Edit: Conversely, her ganks pre-6 are goddamn terrible unless it's an unwarded target that's beyond overextended.
You are already bringing up the laner and her level six ganks. A guaranteed gap closer isn't that great when the enemy can just run away after it's over. It's a very short single target CC with below average range on it.
I mentioned nothing about the laner's damage or CC options. They're always relevant for a gank, however, and to ignore them completely is completely wrong. If we're going to do that, then a lot of amazing gankers become terrible due to the lack of their damage output (e.g., Maokai won't 1v1 nearly anyone).
Besides that, how are they just running away from her? She's on the opposite side of them so if they Flash over her, they're easy to pursue due to wasting half the Flash range. Or they have to run at a bad angle since, again, Vi is now in their retreat path. Red buff or Phage is a slow and they're not escaping. She has auto-attack resets so she's going to be getting multiple autos in once a single slow lands.
And "below average" range on her ult? It's 700. You bring up Maokai and his is 650.
Let me give Vi a prize. Picking off stragglers. Nobody else in League can do that without having to blow fairly long CD ults.
Come late game, it doesn't matter that it's a fairly long cooldown (80s, for the record) if it sets up a 4v5. Or gives you an advantage of some kind. And I merely bring it up because you mention she can't catch anyone out, which really isn't the case.
It's one thing to clear lanes, it is another to push them. Without much to escape with Vi cannot push very hard, or contribute to team fights should they be forced in the way other champions can.
She can escape with her Q. Unless she's caught out and boxed in. In which case, I suppose the same argument could be applied to everyone but the best split pushers, and even then it's fairly shaky for some of them to get out.
Vi clears waves really quickly. She takes down turrets quickly due to her auto-attack reset. She has an okay chance at escaping due to the distance she can get with her Q / the walls she can jump with it. Is she an amazing split pusher? No. Can she push a wave and put damage on a tower and have a shot at escaping? Yes.
I've only have played her in ARAM, but that Q needs to charge up faster....
1s would be amazing for it but it currently takes 1.25s to get to 725 Range (900 collision range). It takes about half a second of charge time for her to dash farther than Flash, even counting the self-slow.