I think you may be over thinking it. I haven't read everyone elses repsonses so excuse me if i repeat something. But I think you will be finr telling them you are on vacation. - How do they know you aren't planning on going to Brazil or Chile, or Uruguay next? When i moved I came in on a one way ticket i don't remember anyone asking me anything. I have also left and returned on round trip tickets ( Back to BA).
I think you will be fine.