I think you are correct that "Vale homo qui es faba" would be grammatical.
A noun following "Vale" is frequently in the vocative form, sometimes in the nominative form, based on examples I have seen from searching the PHI corpus (the correspondence of M. Cornelius Fronto especially seems to contain many examples of nouns in this context).
Here is an example I saw where a second-person verb is used in a "qui" relative clause even after a noun phrase with nominative form:
Vale, meus magister, qui merito apud
animum meum omnis omni re praevenis. (Letter 22)
After a vocative (which "homo" in your example could well be), I would intuitively feel even more certain about using the second person for a verb. The pater noster is of course an extremely famous text where a second-person verb is used in a relative clause after a vocative: "Pater noster qui es in caelis".
Another example of a second-person verb in a relative clause after a vocative is found in Cicero's De Divinatione:
O sancte Apollo, qui umbilicum certum terrarum obtines ( 2.115.2)
(found using the search for #o# ~ qui# that cmw linked to in a comment; Cicero here is quoting some unknown poet)
I can't say for sure that a third-person verb in this context would be ungrammatical, but I did not see any example of it.
The comments brought up a similar structure which is a little less clear to me: Anonym mentions English "you're a man who's a bean", where the direct antecedent of the relative pronoun qui is a noun phrase ("a man") serving as the predicative complement in a matrix clause where the second-person pronoun ("you") is the subject. I'm not sure whether a second-person verb be used in Latin in a sentence like "tu es homo qui es(t) faba". Because "homo" in this context plays the syntactic role in the matrix clause of predicative complement to es, we would not expect it to be possible to interpret it as a vocative (as indicated in brianpck's question about the form macte). Still, based on the "meus magister, qui ... praevenis" example cited above, I would guess that second person agreement would be possible and probably preferred in Latin in this context as well as in contexts like "Vale homo qui es faba".