system
Given the following post, respond as if you're commenting on Reddit:
human
[Witcher] How many higher vampires have been successfully killed?

According to the Witcher bestiary, "the number of higher vampires killed can be counted on one hand." With the exception of Geralt, what other Witchers managed to kill a higher vampire?
gpt
I believe the quote is *actually* "Those who have faced them in combat and survived can be counted on one hand.". It's extremely unclear as to whether any Witchers or humans (besides Geralt) have managed to slay a Higher Vampire. There are only 6 *known* Higher Vampires in existence, and Geralt is responsible for killing 3 (possibly 5 depending on quest choices and alternative media). The only Higher Vampire known to not be slain as of the end of Geralt's adventures is The Unseen Elder, who is powerful enough to instantly kill Geralt before the latter can react. 

Due to their rarity, power and limited infamy, you can assume that most Higher Vampires dwell in secrecy. Hell some Higher Vampires can even elude the Witcher Medallion and pass for human. As Witchers tend to take up all contracts involving murders that Higher Vampires would likely be responsible for (and the lack of dead Witchers suggests the HVs haven't engaged Witchers in combat frequently), it can be suggested that Geralt is one of the few Witchers in recent years (or even decades or centuries) that discovers and battles Higher Vampires. 

It's problematic that Katakans, Bruxae and Alps are *also* classed as Higher Vampires (and would pollute the pool of those who have killed Higher Vampires to include those who have slain the above other Vampires). As for Higher Vampires as we would know them, we can only guess. There isn't any source material explaining who else besides Geralt has tangled with a Higher Vampire and lived, let alone slain one. 

I'd suggest Vesemir might be up there, and Lambert's familiarity with Vampires like Ekimmaras could suggest he's been able to track down and slay some Higher Vampires. It's all theorising though, as Geralt is noted by even the Higher Vampires themselves for managing to uncover their true nature *and* slay some of their number despite the many generations of power they have built up.