1st and 2nd evolution: These two evolutions only require a close enough bond with their owner, a determined amount of time in training has to be recorded at this point. This is to keep Vivora from overpowering themselves if they ever abondon their owner, so this keeps things stable. (There are a couple exceptions of where betrayal has happened beyond these evolutions)
3rd and possible 4th: These evolutions require power, meaning.. well, for them to evolve to the third evolution, they must defeat about 9 Vivora bearers. Or in other cases.. beat Vivora bearers 9 times if there is a limitation of Vivora (only a few who own them who are around). When it comes to the fourth evolution, they have to defeat Vivora bearers 30 times- and the fights from previous evolutions count. You don't have to ask me to evolve them at the third and fourth, their evolution will be determined by their winnings. IF the Vivora happens to betray their owner and any time point whilst in their 3rd or 4th evolution, their forms would change into a more gas-like formation. As their negative energy has overtaken them entirely.
(There is a possibility for the 3rd or 4th evolved stage Vivora to possess their owners. This would require consuming immense amounts of their own negative energy and replacing it with their owners energy. Some cases, their power can be exceedingly destructive.. and if any Vivora attains a certain gene directly from their energy, it's possible for them to resemble their owner, but their own personality and powers will remain intact. Some say that this gene is either from an extinct Ominorian virus, or another Vivora passed away who contains the gene. The energy is like a chain, some Vivora are connected to others.)