Posto che se non erro la grafia corretta sarebbe va be', in caso per qualche motivo lo si debba scrivere in questa forma gergale, l'accento andrebbe grave (vabbè) o acuto (vabbé)?
This question is not simple to answer to. Indeed this term is the short form of "va bene" (that's ok), and is rarely used in written texts.
But if you really want to write it, I would choose among three different forms:
- vabbè which is how you pronounce it
- vabbe' which is the linguistical correct form, with the truncation of "ne"
- vabbeh which correctly represents the pronountiation (aspirated 'h' sound)
As pointed out by Daniele B., the correct spelling is vabbè -- which is the correct usage if you want to infer a (southern) dialectal inflection.
That said, if you want to simply use a gergal, yet dialect-transparent form, there exist three correct spellings:
- va beh
- va be'
- va be
All are mentioned in this Accademia della Crusca thread.
Often seen, but incorrect, is the misspelled version
- va bhe
The online Treccani dictionary does not list an entry for this expression. It does list an entry for the word
be' (which is an abbreviation of bene which according to this dictionary has apparently made it into standard Italian, and I have quoted the online reference below):
be’ ‹bè› (o beh) interiez. – Troncamento fam. di bene, usato, con il senso di «bene, ebbene», in frasi interrogative o di concessione: be’, che succede?; beh! ... fate voi.
Nevertheless, I wouldn't write "va be'", which is two words, because the b is pronounced as a double sound and since (1) in Italian double sounds only occur in the middle of words (they cannot occur at the beginning or at the end of a word like in other languages), and since, just as important, (2) the given word is part of nonstandard Italian, one can conclude that the best to write it as it is pronounced, ignoring everything else. Hence, IMHO, the most proper way to write this word is: