"Do any scriptures say that the hunter Jara was a rebirth of Vali? Are there any scriptures that describe the previous birth of Jara?"
The simple answer is, NO. [As per this meta post].
No authentic scripture discuss the previous birth[s] of Jara. There are only folklores around VAli & Jara. Some even believe VAli got rebirth as Arjuna, by retaining the lineage of Indra.
Our creative imagination relate 2 personalities. It may be right, may be wrong. Example,
Even though, Jara was a too insignificant character compared to extremely courageous VAli, the main source of this legend has to do with: VAli avenging his death from RAma, by getting reborn as Jara and killing RAma's own rebirth Krishna.
But what if, "Krishna" was not the rebirth of "RAma"? In such case, the whole theory of VAli-Jara will collapse on its own.
Previous births of Krishna
In sacred-texts, below previous births of Krishna are described, which are [mostly] prior to RAmAyana:
- As a portion of Vishnu: From Adi Parva,
And he, called Vasudeva, endued with great valour, was among men a portion of him called Narayana (translate Vishnu) --the god of gods--eternal.
- As a son of Dharma: From Drona Parva (also the source of point-3),
He that is called Narayana is older than the oldest ones. For accomplishing some purpose, that creator of the universe took his birth as the son of Dharma. On the mountain of Himavat he underwent the severest ascetic austerities.
- As a Rishi NArAyana: From Karna Parva,
When the two Krishnas (Arjuna & VAsudeva) are excited with wrath, they show regard for nothing. These two bulls among beings are the Creators of all real and unreal things. These two are Nara and Narayana, the two ancient and best of Rishis.
RAma is nowhere mentioned as Krishna's previous birth.
Not to be confused with Gita 10.31, "Among wielders of weapons, I am RAma". Because these [chapter-10] forms are from the supreme Brahman perspective and not Krishna's personal form perspective. Moreover, it can mean Shri RAma, Parashu RAma & even BalarAma as well.
Supportive reference: Like how RAma & Krishna are not related, similarly RAvana & ShishupAla also are not related [as per Jaya-Vijaya theory]. :
From Adi Parva:
that son of Diti, who was known as Hiranyakasipu, was known in this world among men as the powerful Sisupala.
From SabhA Parva:
Bhishma said,--The will under which the ruler of Chedi (Sisupala) summoneth thee to fight though thou art of strength that knoweth no deterioration, is scarcely his own intention. Assuredly, this is the purpose of Krishna himself, the lord of the universe. ... This mighty-armed one is, without doubt, a portion of Hari's energy. And surely, the Lord desireth to take back unto himself that energy of his own.
RAma himself might not had rebirth
If we name the supreme Brahman as "Vishnu", then RAma & Krishna can as well be different portions of "Vishnu", like ParashuRAma. They need not belong to same lineage of births.
RAma is believed to have taken SamAdhi in Sarayu river. This SamAdhi itself would have resulted in the state of no return for RAma's own consciousness [Gita 3.4].
[Similarly VAli also could have got Moksha, after being killed from the hands of RAma.]
Summary: Since RAma & Krishna may not be related by rebirth, there are no sufficient proofs to link VAli & Jara by rebirth. Hence, VAli & Jara were different consciousness.