First, if you haven't already, I'd recommend checking both buns over very thoroughly, including the belly, as it can be very easy to miss injuries hidden in their fur.
If one wasn't feeling well post vaccination, that could be enough to spark something. Sometimes when a bun is in pain or feeling unwell, it can cause them to show reactive aggression towards the other bun.
The problem with a bonded pair having a serious fight, is there's a chance they won't rebond. And from the sound of it, it does sound like there was extreme aggression. So I feel I also need to add this caution, that the behavior of going for the belly with teeth (or claws), is an attempt to inflict serious, possibly fatal, damage to the other bun. If you feel they're to the point already of being too aggressive and reactive towards each other, it may be the point of no return already, and no rebonding should be attemped or it risks serious injury occurring.
If you do feel there's a chance and would like to attempt to rebond them, I'd suggest keeping them completely separate for at least a couple weeks. They essentially need time to forget the fighting that's occurred.
Then if you want, try a brief highly supervised interaction, to see if they're ready to be rebonded. But be ready to immediately intervene and separate, carefully with a soft broom or gloved hand, etc. You just don't want to get bit yourself or risk harming them, but also need to be ready to try and prevent a potential fight and injuries occurring.
If they start up with escalating aggressive behavior, I would intervene immediately and keep them separated several more weeks before trying again.
If they're showing any aggressive behavior between the pen bars of their separated areas, you may even need to put up cardboard or other physical barrier, to block sight and contact, or even remove one bun to a separate room. They'll need time to forget each other and the fighting that's already occurred.
But even given time, there are no guarantees. Some buns, once they've had a real fight (not just minor bonding dominance behavior), may not ever rebond. And the more aggression and fighting that's occurred previously, will further diminish the chances of a successful bond occurring.
I'd suggest refreshing yourself on bonding and the signs of what escalating aggression looks like. Sometimes buns will start to become slightly aggressive, but with good intervention it can be stopped before it escalates into an all out fight, where bonding then has to cease. Because once that fight occurs, chances are high bonding won't be successful at any point.
The hope though, is that this was prompted by the vaccination and that they'll be inclined to rebond once any ill effects have worn off. But that becomes less likely each time aggression and fighting is allowed to occur.
https://rabbitsindoors.weebly.com/bonding-bunnies.html
https://cottontails-rescue.org.uk/information/bonding-bunnies/
https://wabbitwiki.com/wiki/Bonding_rabbits_together