Literal translation: Vamos a hablar pronto, hablemos pronto. But I prefer: Cuando puedas, hablemos. Tenemos que hablar un día de estos. Cuando te venga bien, hablamos.

let's talk soon = hablemos pronto!, let's talk on sunday = hablemos el domingo! using like a presice time as you said

Well "let's talk about soon" means will be in touch, it's more general. you do not say the exactly day,the time, but if you say "let's talk next Sunday or on Sunday" you are saying when. En Español tambien decimos "nos hablamos",no indica cuando, por otrolado "hablamos el Domingo que viene" y especificamos. Regards!! Carla

you shoud say "hablamos pronto" ó, "hablamos al rato", ó "hablamos luego". Anyway, as you use it probably as a farewell, this means not exactly that you want or have to talk to this person, is more a sample of good manners when you say good bye, than a real meeting, much diferent from saying "let's talk on sunday", or "I'll talk to you on sunday", "hablamos el domingo", which is more for real purposes of talking to someone in that specific day. So, it's important to know the communicative difference considering the intentions of the speakers, wanting to say good bye, arrenging an appointment, a meeting or a date. Sometimes, people use Let's talk soon when they have a real need to talk to someone, but dont know exactly when they can do it, but finally the speakers have to arrange it and say ok, let's talk on sunday, in you house maybe, etc. regards.

Hablamos al rato.