How much better is flying online with AS compared to the regular weather from VATSIM?And does it work well for VATSIM (disabling SB weather updates etc.)?

The SB2 weather interface is rather old and it isn't made for FS2004. You must remember that the controllers don't have access to the weather that you see in FS they extrapolate the conditions from the current metar so is rather important that the metar description be accurate. The only thing that you should take in consideration is the TAF processing it should be off in VATSIM flights and of course choose VATSIM metars.Jos

so AS can take the METAR's from VATSIM? and does it work well? :)

Yes it can take the metars and it works well pending there are no VATSIM server problems.Jos