I don't see why it wouldn't be localized. Namco decided to localize Eternal Sonata, afterall. Look at that game's sales:
http://www.vgchartz.com/swlaunch.php?reg1=Japan&game1=Eternal+Sonata+-+PS3®2=America&game2=Eternal+Sonata+-+PS3®3=All&game3=&weeks=50
Its NA sales will pass up the Japanese sales in the next few months.
Much like Eternal Sonata, Namco could probably expect a similar amount of sales from a North American release of Vesperia as they could from a Japanese release. Why port the game for an extra 100k in sales but not bother localizing what amounts to only half a game for a similar amount? The costs of finishing the localization (continuing where the 360 version left off) are far less than those of porting the game, I'm sure.
