That's a very standard expression, not slang at all.
Embora is an adverb there meaning "fora daqui". You can even omit
vamos, and say only
Embora!, using it as an interjection, though that is more likely to mean Vai-te/Vá-se/Vão-se/Ide-vos embora.
Some times though, in informal use,
embora can be used beside
vamos without having any meaning (just as a modal particle indicating encouragement -- hortative modality).
Vamos can also be omitted and
embora shortened to
bora (even more informal):
Vamos lá embora comer!
In the first sentence,
embora could be omitted without much being lost, the sentence would be just a little weaker. On the other hand,
lá adds a touch of impatience, though not as much as in Let's eat already.
Dictionaries also tell us that, as an adverb, it can mean "em boa hora" (the locution whence it originally came), but I don't think I've ever heard it as such. Aulete marks it as obsolescent, but not Priberam. Do not confuse it qith
embora used as a conjunction, which expresses concession.