"There is someone out there."
Translation:Có một ai đó đang ở ngoài kia.
There is a subtle difference in that "bên" is more-so referring to the side that an object is on while "ở" is more-of where something is located. Outside of that they're usually the same in meaning no pun intended.
Can đó also refer to "yonder", or are này, đó, and kia more like ここ, そこ, あそこ in Japanese or aqui, alli, alla in Spanish?
Both đó (Southern) and đấy (Northern) are used to mean a generic "there" or "that"; kia is usually more ambiguous/unspecified and refers to the "other" thing. There are some cases where you have to use kia instead like "bên kia đường" (on the other side of the road) or "bên kia sông" (on the other side of the river).
For example if someone asks where the bank is and it's within view you can say "ngân hàng (nhà băng = used mostly by overseas Vietnamese) (nằm = to be located) bên đó/đấy/kia. There's also "kìa" which acts kind of like "voilà" with the opposite (closer equivalent) being "nè". However, if you're pointing at a particular building you can refer to it as "nhà đó/đấy" while "nhà kia" is more of "that other house" that may or may not be present or simply implied.
which is more commonly used in everyday language - "đang ở ngoài" or just "ở ngoài"? and do they have any different shades of meaning? thanks :)
đang just means "in the process of" or "currently". If someone asks "bạn đang ở đâu" you can omit "đang" in the reply since it's implied already.