It's not that the "estas" is optional. "Malprava" without the "s" is an adjective. So you can't say "vi malprava", because then there is no verb in the sentence. "Malpravas" with an "s" on the end is a verb in the present tense, that is, it already carries the sentiment of "are", so you don't need another verb. So you can say either "Vi estas malpravA" or "Vi malpravAS" and it means exactly the same thing. A more experienced Esperanto speaker than I might be able to say whether there is a semantic difference between the two, but if there is it is very subtle. In general I think it is preferred in Esperanto to be as concise and agglutinative as possible, so always go for the option that uses the fewest words.