Danny... Phantom? Don't see the connection...
Hmm, I'd say that Trigun and Rouroni Kenshin are VERY alike... Actually, I'd have to say Rouroni Kenshin is pretty much Trigun in feudal Japan. (Alright, so it's actually Meiji Restoration, but Feudal Japan is easier for people to recognize). Kenshin and Vash are very much the same, two idiots who refuse to kill who have a bad background (The difference being that Kenshin actually DID kill in the past and his reputation was deserved and he wants to reform, while Vash never did anything).
Also, Slayers is a lot like Trigun. It never EVER takes itself seriously, and while only the beginning of Trigun is like that, and it gets more serious later on, Vash himself tends to not take things seriously a lot, joking around in the most dangerous situations. Apparently, in the manga, he was fooling around in the final battle with Knives, and I think people were offended by it or something, or perhaps Nightow though it was offensive, because he removed "The offending material". Come on, why'd ya do that? Back to the Slayers, I find the humor to be very Trigun-esque, and well, yeah... Like Kenshin is Trigun in feudal Japan, Slayers is Trigun in a medieval fantasy setting.
As for a series that merely have characters that are like Vash:
Axis Powers Hetalia. I swear, Italy is just exactly like Vash, but instead of donuts, he loves pasta. And Italy is so freaking moe! His voice itself is very moe. I think his voice would have been a better fit for Japanese Vash than the one Masaya gave him (JYB's Vash voice is indeed moe enough for him). Yes, I'd have to say Vash (and Italy and Kaito, a Vocaloid) are one of the few male characters that aren't shota that can be described as moe. Anyway, back to Hetalia. Well, Germany can also be compared to Wolfwood a bit, too. To give you an idea of how Vash-like and adorable Italy is, I present to you an episode of Axis Powers Hetalia (They're only five minutes long and the Hetalia anime is officially streamed on the web, so these fansubs are completely legal):
This is the third episode, and it's a perfect example of Italy's Vash-ness Who knows, maybe you guys'll start liking Hetalia!