The new vendor weapons, including factions, will have a perk way over on the left side that looks like 2 crossed hammers, and these can all be reforged via the gunsmith for a somewhat nominal fee. Only new weapons will have the re-roll perk. So no to current and raid weapons, but some of the new ones are re-skins... Like they showed Red Hand IX available for sale in HoW at New Monarchy, and it will have the perk. Works just like all iron banner weapons now. Essentially all HoW vendor weapons will work like all Iron Banner do now, except you go to the gunsmith instead of lord Saladin. Re-rolling resets the weapon completely as if you just bought it again. They did not show an ascended weapon being re-rolled, but the current model is that it removes all XP and perks upon re-roll.