What is the correct or usual way to respond to Brazilian greetings?
I've been greeted with Como vaí, E aí, and Como está but I'm not sure what the proper or common response is.

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usually speak (informal forms):
1) E aí, beleza?
2) E aí, tudo bem ?
3) Beleza?
4) Como vc tá?

in English would be like "Hey, what's up?"

the answer may be the tractional you've likely learned.. like: "Estou bem", "Mais ou menos", "Estou mal" etc.

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Estou bem, obrigado(a). E você?

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So... "como vai?" and "como está?" have the same meaning: "How are you?"
In Portuguese it's usual to answer these questions with "beleza", or "tudo bem, e você?"... it's kinda like in English.
"E aí" is the Brazilian version of "What's up?" haha, then you can answer like Denis said "Estou bem", "Mais ou menos", (...), "Não tão bem"...

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vc = você

*sorry for abbreviating