I'm talking about Mr. Blonde (Reservoir Dogs) and Vincent Vega ( Pulpfiction).
They both have the same last names. Are they brothers or not?

No. They are lovers.

Yeah they are, henceforth all the rumors about the "Vega Brothers" movie with Travolta and Madsen. Granted, that'll prolly never happen and them being brothers has zero relevance to the movies whatsoever. But in Tarantino's mind they're brothers.

i thought tarantino wanted madsen to play the role in Pulp fiction but he was busy, so he changed the first name and cast travolta.

am i wrong ?

Well not exactly, I believe it was always Vincent in Pulp, he just wanted Madsen to play his character's (from RD) brother.

I know Quentin originally wanted to do a Vega Brothers movie as a prequel to Reservoir Dogs and Pulp Fiction but time and the aging process put the kabosh on it. So why not just set it after Reservoir Dogs and Pulp Fiction? How can you do that if they're dead, you say? Well, the reality the movies exist in is slightly askew from the real world anyway so why can't Tarantino make it so that each brother survived their encounter with bullets? Perhaps he could even turn it into Pulp Fiction 2 and tell the Vega Brothers story while weaving in the continuing stories of the characters from the first. Imagine Vincent and Vic going to the movies to see Kill Bill, the big screen version of the failed television pilot Fox Force Five, starring Mia Wallace or Jules meeting up with Vincent, while walking the Earth, but this time they're on opposing sides. The possibilities are endless.

What about the Scagnetti brothers?
(Vic Vega's parole officer in Resevoir Dogs/ Cop hunting Mickey & Mallory in Natural Born Killers)

When Travolta was on "Inside the Actor's Studio" they made it seem like he was the one he always wanted for the role.

There is a lot of talk about a Vega Brothers film. Yes, they are brothers. But I'm in the camp that want to see another Gecko brothers film just for Seth's tattoo.

in reservoir dogs, there is a line "so, how's Alabama been doing". is that refering to Alabama Worley in True Romance?

Yes it is, but it's a strange one, because it coincides more with the script for True Romance than the actual movie.