Closing in on two decades playing this damn system and I still find holes in my knowledge on the rules sometimes, sheesh.

I was making an NPC that really had a thing for knives. As such, I paid more attention to the knife selection in 3e than I ever had before, and noticed something strange: vibro knives aren't any better than your normal Cougar Fineblade. Hell, the Fineblade actually has slightly better concealability and weight. The damage is the same, the availability is the same...the Fineblade is technically cheaper, but with a higher street index. Which of course means getting it on the black market is slightly more expensive, but by like...400 nuyen.

Meanwhile, the vibro-knife ruins stealth and forces you to worry about things like battery life and purchasing new ones. Are there some generic Vibro quality rules for the edition that I'm just...missing?

Are there any rules for the vibro knife? No, not that I'm aware of. And the downsides are just as you stated. What I would do is give the wielder of the knife a bonus on interrogation / intimidation checks, just based on a bone shaking whining noise coming from the knife of death being waved in the air. I mean sure, having someone pull a knife on you if you're the average person is all kinds of scary. But having a knife that vibrates so fast and hard that it'll slice flesh from bones and sever limbs (at least according to the trid shows and the safety warnings) - that's got to make you nervous.

Personally I'd look at the situation and see when and if a bonus might be appropriate. Thrusting or slashing with a knife in an underwater combat is less effective than on land due to the water resistance - I'd probably not penalise a vibroknife though (as long as it was water sealed and didn't short out of course!) due to the nature of how it cuts.
As mentioned above, I'd give a reasonable modifier to intimidation and interrogation checks against people being threatened with a vibroknife. Equally I'd want WP checks for PCs when they come up against a whole gang waving vibroknives around, forming some kind of evil harmonic shrieking that sets the teeth on edge, because that has GOT to be unnerving.
Somewhat similar to the first one, if the combat was taking place in some situation where you couldn't get a full swing, I'd penalise a normal weapon and not a vibroknife, which isn't relying entirely on angular or thrusting momentum to do the damage.

But on the whole, they're not fantastic. I've thrown them into my game, and one of the players picked it up purely for the threat aura / cool factor. I don't think he's used it in anger yet...

It's an electric carving knife, right? I don't see anything in its CC description preventing it being dikoted.

Which still wouldn't make it "better" in any way when compared against another dikoted knife. In other words: vibro knives in SR3 are largely a cyberpunk trope where "style-over-substance" has to suffice when compared against most other forms of knives.