Both are correct but depends on how`s the meaning you want to ask for in Portuguese language. So, look for the explanation below:
If you want to know how is the personal feelings of anyone, grammatically correct to use in Portuguese is " como vai voce?", just in english " How are you? . The other expression means that you ask someone about "How`s the way he/she will take to go to some place or other mean to "como voce vai?" is " What will you dress to for....?".
Understanding:
1) Como vai voce? ( Interrogative Pronoun+ VERB+ Personal Pronoun or Subject+ complement). In English You ask People personal feeling ways inthat moment ( Good, bad, moreless, excellente,etc) .
2) Como voce vai? In English, you are asking for transporting way, what dressing clothes to go for some place, as well.
3) You can find in Portuguese a lot of expressions but only used informally or daily.
Examples:
How are you? ( Como vais? / Como estáis?/ Como estáis tú? / Como estao as coisas? / e aí? ( E aí is similiar with " what´s up") and more.
And all the other explications that our friends have done above are very good and well considered.
Best Regards,