This is indirect, but the Klingon word for "Vulcan" is vulqangan, and ENT established that the Klingons met Vulcans before they met humans. So presumably the Klingon word came from the Vulcans themselves. Although that's admittedly a retcon, since the Klingon word was coined by Marc Okrand decades before ENT.
At least it's not as bad as with the Romulans. It's obvious that "Romulus and Remus" were meant to be human-assigned names for the twin planets, based on the famous mythological twins. But the writers of ENT: "Minefield" must not have known their mythology, since they had T'Pol explicitly say that the Romulans' own name for themselves was pronounced "Romulan." That was a scream-inducing moment.
At least it's not as bad as with the Romulans. It's obvious that "Romulus and Remus" were meant to be human-assigned names for the twin planets, based on the famous mythological twins. But the writers of ENT: "Minefield" must not have known their mythology, since they had T'Pol explicitly say that the Romulans' own name for themselves was pronounced "Romulan." That was a scream-inducing moment.