There is nothing at the Legendary level that can really compare to this Regeneration edge you are proposing. (There is a monstrous ability called Regeneration that is very similar, but monstrous abilities follow different design goals so I skip it for now and talk about it below.) The most similar edges (on all ranks) in the core rules are:
- Fast Healer (Novice, Vigor d8): +2 to natural healing
- Hard to Kill (WC, Novice, Spirit d8): ignore wound penalties for "death roll"
- Harder to Kill (Veteran, Hard to Kill): 50% chance of "not dying"
An edge that allows you to roll Vigor each turn to heal wounds is more powerful of any of these, so it should be at least Heroic. If it does not stop working after the character is dead (making the character effectively immortal), it is way more powerful than any of these edges, or any other edge in the core rules for that matter.
The way you wrote it, there is no way you can balance that edge with the other Legendary edges in the core rules: it is much better than any of them, or even several of them put together. I can only think of two strategies to balance it a bit (not much), hopefully one of them will be OK for your campaign.
Make it expensive
The first way of balancing a powerful edge is splitting the edge into several edges, so that it is expensive to get. For example, having four steps of "regeneration" could look like this:
- Basic Regeneration (WC, Legendary, DM´s call): If the character has one wound but no more, she can do a Vigor roll every round. If the roll is successful, the character heals the wound; if the character has two or more wounds, the regeneration stops working until the character has one wound only. Additionally, the character makes natural healing rolls every three days (instead of five) regardless of her number of wounds.
- Advanced Regeneration (WC, Legendary, Basic Regeneration, DM´s call): Same as Basic Regeneration, but it does not stop working unless the character has three wounds or is incapacitated. Additionally, the character gains a +2 on natural healing rolls, regardless of her number of wounds (this does not stack with Fast Healer).
- Enhanced Regeneration (WC, Legendary, Advanced Regeneration, DM´s call): Same as Advanced Regeneration, but it does not stop working unless the character is incapacitated. Additionally, the character makes natural healing rolls every day (instead of every three) regardless of her number of wounds.
- Full Regeneration (WC, Legendary, Enhanced Regeneration, DM´s call): Same as Enhanced Regeneration, but it never stops working, not even if the character is dead. As soon as they heal at least one wound, they stop being dead / incapacitated and come back to life.
Some of these steps share ideas from the monstrous ability Regeneration. Even if these are four edges instead of one, any of them is still clearly better than any other Legendary edge, which brings me to my second option.
A common trope in most stories is that very powerful beings have some terrible vulnerability. Most vampires die horribly under sunlight, many types of zombies (and in particular those in Savage Worlds) are most vulnerable in the head, werewolves cannot regenerate wounds made by silver bullets, some trolls regenerate unless wounded by fire and acid, Smaug had a tiny hole in his armour, etc.
Your character looks like she is effectively becoming one of the most powerful characters in the story, so in return of regeneration she may gain some special vulnerability. Maybe her head becomes especially brittle (+4 to damage on head shots), or she cannot regenerate some kinds of wounds (e.g. silver weapons, fire, acid, salt, wounds on the head, wounds done in the dark), or any other vulnerability that fits the theme and the campaign.
For comparison, this possibility is explicitly included in the monstrous ability Fast Regeneration: "Most creatures with this ability also have a Weakness or Vulnerability, such as fire. Wounds from the creature’s Weakness or Vulnerability do not regenerate, but may still heal naturally".