Tub surround help.

Hi. I'm new here on this forum . I've been pulling my hair out with indecisions, so I thought I'd ask here. I gutted out my bathroom, and am installing a new tub and using a 5 piece tub surround. My questions are ,

Can I install greenboard first,place in the tub and attach to studs, then install the surround? Or, do I install the tub directly to the studs, bring wall board down to flange of the tub, then cover with the surround? I feel this isnt right because the flange obviously isnt the same thickness as the drywall.Some guy told me to bring the board over the flange and caulk.??? A correct procedure using the surround will be appreciated! Is the a vapor barrior needed behind the green board? Thanks in advance!! BTW, This is a tub/shower combo.

Tub goeis if first. Greenboard down to the flange, then the surround comes down to within a 1/4" of the tub deck. If you fir the studs out, you can bring the greenboard lower, but calk well because you do not want it to ever be able to wick water.

Thanks. Is there a problem with doing the walls first, then placing the tub? It would make the surround go right down the wall and smoothly over the flange. Wouldnt the way you describe leave a void between the flange and back of the bottom edges of the surround,since they are different thickness's? I'm just trying to understand the logic to doing it that way.Thanks.