Both have fought the likes of General Grievous, and both have faced Palpatine's apprentice of their respective eras. Vos as traded victories with Count Dooku in Dark Disciple, while Eeth Koth put up a solid fight against Darth Vader.
Koth fought Vader only shortly after ROTS IIRC. Peak Vader is at around the time of Rebels (maybe the OT). But Vader at that point is still adapting not only to the suit but to being a Sith.
Koth's death was toward the end of the 2017 Vader run, implying Vader had been in the suit for around 3 years, if I remember the timeline correctly (Koth's death was one of the inciting events that led to Vader relocating to Mustafar). By the time Koth had died, Vader and the Inquisitors had already hunted down very nearly every surviving Jedi in the galaxy.
So I wouldn't say that Vader was still adapting to the suit by that point, but I do agree he wasn't at peak yet. That said, he'd still be around as powerful when he fought Koth as he was in Fallen Order, so still an absolute powerhouse.
Insider #199 states that Vader killed Koth ~5 years after Order 66.
So while Vader may not have been at his peak when he battled Koth, he would have still been well-accustomed to his suit, and in all likelihood, above the likes of Dooku at that point(since canon explicitly notes that Vader's power/skill did nothing but increase after the events of RotS, and that he was Palpatine's most powerful apprentice.)
That being said, Koth should logically be able to put up a very good fight against Vos. Don't know if I'm comfortable saying that he could win, though, as his only feat is contending decently against Vader before getting killed.
That's true, but there also weren't a whole lot of notable Jedi still floating around during this era(that Vader knew the locations of, at least)... But we can't really hold that against Vader, given the numerous accolades which place him above basically everyone this side of Palpatine.
The thing that still blows me away about canon Vader is that he has virtually no low showings across all media. He has been consistently wanked in almost every appearance- from books, to games, to comics, to TV shows. It's extremely rare for ANY fictional character to get that kind of respect; especially with the dozens upon dozens of appearances Vader has racked up since 2014.
Shit, I think his only legitimate "loss" in canon was against Sidious himself, who is just on a completely different level, and as close to truly unbeatable as this genre can provide.
It's ok to have low showings. Messi is the best player in the world but even he sometimes has some bad games. Unless you're a robot you're not always going to perform at 100% in any scenario.