Sometimes vendors that are standing on upper floor levels cant hear you talking if you are standing downstairs on the ground level/street and wont turn to face you on spoken commands.
In that case stand anywhere outside the house and open their customize gump from the context menu.
They will jump outside onto your tile, leaving behind the infamous red reserved sign at their spot.
Dont close the customize menu yet, make one step away from the tile in the direction you want them to face and then cancel/close the customize menu.
They will issue some witty comment about good sales or stuff and will always turn and face you.
Just return them to their spot with the context menu. Or just leave, they will jump back inside automatically after 1 minute, too.