Is Vash really worth 4-5 points though?

Well the series and manga are based around him, he's the main character, the story revolves him, most praise from the series comes from his character, I would say that in my honest opinion yes. He held up Trigun, he is the highlight of the show.

it seems like the author looked at Ruroni Kenshin and said "hm, how about I make this series, but set in the future."

Kenshin wasn't anything new either, Batman probably the earliest example (that I can think of) of the kind of character Vash and Kenshin would be, dark past, trying to atone for their sins (In the case of Kenshin for himself, while Vash for Knives), and their personality really doesn't give off such a darkness to them. I believe Trigun different enough to not really to be called Kenshin ripoff, it feels very western inspired while Kenshin is more inspired by Japanese history, the stories and set up are different, and really the only thing comparable feels to be that Vash and Kenshin are somewhat similar and came out around the same time for their manga.

I liked Badlands Rumble, not for the plot (which was a mediocre anime movie plot), but because the animation allowed the world of Trigun to be fully realized.

I agree with that, the animation highly improves and shows off the world better, but for me what I liked with Trigun was the story and characters, they are what made Trigun, with a mediocre western world that was seemly limited with Madhouse's budget at that time. But that's almost it for what I can say about the film, none of the characters that strong in a stand alone, unlike the series which mostly (not all the time) avoided quite a lot of cliches from that time while the film was just full of them,

You've gotta admit, the animation for the tv series is really bad

It's not the worst at that time, but yes, comparing to Bebop or Outlaw Star (came out same time) it definitely doesn't hold up in terms of animation.

and the tonal shifts felt really drastic to me.

That's part of what makes it likable to be, I think comparingly to Kyoukai no Kanata for instance, Trigun for me had that good balance of comedy and dark action/drama, they were drastic but they fit Vash's character, he's silly but normally trying to detract what he's hiding with his own past and not wanting others to get involved and more specifically killed.

And just for information, I'm not saying that Trigun is a masterpiece, it's not, it's simply a favorite of mine. I would not be willing to argue it's on the same level of Ping Pong: The Animation, Revolutionary Girl Utena, or Spirited Away, because it's not, it's just a favorite.