If I could possibly resolve this as best I can, I do remember just a bit of stuff about Dragon Ball that will make sense.
Civilizations rise and fall when its people multiply and eventually goes extinct. We have to remember that the planet in question wasn't Vegeta in the first place, it was called Plant. We just saw the original inhabitants of the planet and it's quite possible that when Chilled's forces came by or some other evil expeditionary force arrived, something happened to the original civilization and the Saiyans eventually rose to become the newest race on top of the planet (Though you can also argue that the Saiyans may have gone to war against the Plants and eventually took over the planet once they started to evolve).
It took them lots and lots of years, but the Saiyans eventually claimed the planet. Then the Tuffles came along and there was a bit of an uneasy coexistence during that time. Then the Saiyans decided that they'd wanted the planet for themselves for real and then things got along as well as all of you can expect. Then they rechristianzed the planet from Plant to Vegeta in honor of the ruling family that rose to lead the Saiyans and the rest is history.
As for that last part, yeah, It Was All A Dream, Gohan. You can go back to sleep now.
Those purple things being called "Plants" is a complete fabrication on the DB wiki's part.
They tend to make shit up, I wouldn't go there if you want information about Dragon Ball rather than Blatant Lies.
In the original EoB manga/OVA, those little purple things weren't given a name at all. All we know about them is that they lived on a planet that resembles a primitive Planet Vegeta, and that they have medicine that resembles the fluid used in Freeza's healing tanks.
Tuffles were mentioned in filler, movies and GT. Toriyama came up with the original concept but didn't flesh it out to the extent that the anime did. It's likely that Vegeta isn't actually the Saiyans planet - they're "Saiya-jin" in Japan, which means "People from Saiya". Saiya is their home planet, and they could have either renamed it to Vegeta after defeating the tuffles (what the anime and GT suggest) or left Saiya and invaded what would become Planet Vegeta (what EoB and GT suggest).
Yeah, GT tends to contradict itself like that.
edited 6th Nov '12 2:47:53 PM by Saiga
Hence why Piccolo told Gohan to go back to sleep at the end. The normal/canon world of DBZ doesn't make much more sense, really, and if Gohan keeps thinking about it too hard, then we start going all Tomato in the Mirror route, and he starts waking up from multiple layers of dreams...[[/spoiler
It is like what Gohan said: None of that makes any sense.
—-
- So, here is my explanation: Goku was feeling very curious about his origins. So he went out and asked King Kai about that. Then King Kai tele-casted Bardock's story to Goku and his friend through Kai-o-TV.
- Also, I just figured out there was a legit case of Foreshadowing in the beginning of the movie. When they retell the story of Bardock's death. The only difference is Bardock saying: Crap baskets!, which is really Gohan's Oh, Crap! catchphrase .
edited 7th Nov '12 5:14:51 PM by nairoxev
Yeah, I picked up on that line too, and it was definite Fridge Brilliance.
IRC (And I'm not digging my Mangas out to check), in the Manga it's made clear the Saiyans aren't from Vegeta, but that their original planet was destroyed by the Legendary Super Saiyan. I know the Show actually shows this happening, while in the Manga it's all narration from Vegeta.
... That's a good question. Last we see of him he's just being crowded around by the cheering natives.
You are not alone.

It's a story within a dream within an alternate universe within an adaption that's gone back in time.
There. That explains everything.
You are not alone.