I recently came across a book listing the imperative tú form of the verb valer as val. I also came across a website that agreed; however, when I checked on RAE.es, Wordreference.com, and Spanishdict.com, the conjugation was listed as vale. Is val a mistake or is it an archaic conjugation.
This is how the Nueva Gramática describes it:
El verbo valer poseía un imperativo monosilábico (val), usado generalmente en su variante pronominal. La forma antigua valte equivalía, pues, a la actual válete: Aprieta pues bien con ambos braços tu dicha; que es ella revaladiza como culebre: valte del polvo de tu ser para tenerla asida (Garau, Sabio)
So in effect, yes, the form val existed in the past, but it is no longer used. The RAE's conjugation engine sometimes doesn't include irregular forms even when they're correct (plugo shows up for placer, but not plegue), but they don't ever show archaic verb forms that are no longer considered standard.
I would not use it today. The only people who speak Spanish today that would ever say val are bilingual speakers who also speak Asturian or Catalonian because the verb in the indicative/imperative has that form (and really only when speaking those languages).