Do I need a vapor barrier on tub surround with 1 external wall?
You need more than a vapor barrier...you need waterproofing. Tile and grout and hardi-backer aren't in and of themselves waterproof. 

The easiest way to do this is to use Redgard. You put up the backer board, tape and mud the seams, then you paint the Redgard on. This creates a completely waterproof rubberized waterproofing layer behind your mortar/grout and tile.