2) It might depend on the character. Theoretically, a reincarnate can Echo nearly anything their preincarnation possessed, be it memories, emotions, thoughts, items, powers, creatures, etc. If the preincarnation possessed the ability to go 100% with a power, but chose only to go 75% of the way, then the reincarnate could Echo back the ability to go 100% with a power, but only the memory/knowledge/skill to go 75% of the way with it.
For your example, since Vegeta never reached Super Saiyan 3 in canon, his reincarnation wouldn't be able to Echo back a Super Saiyan 3 state. Depending on how a character progresses from SS2 to SS3 in canon, it may or may not be possible for Vegeta's preincarnation to progress to SS3. (For example, if the progression relies on outside factors or elements not present in STE.)
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2) It might depend on the character. Theoretically, a reincarnate can Echo nearly anything their preincarnation possessed, be it memories, emotions, thoughts, items, powers, creatures, etc. If the preincarnation possessed the ability to go 100% with a power, but chose only to go 75% of the way, then the reincarnate could Echo back the ability to go 100% with a power, but only the memory/knowledge/skill to go 75% of the way with it.
For your example, since Vegeta never reached Super Saiyan 3 in canon, his reincarnation wouldn't be able to Echo back a Super Saiyan 3 state. Depending on how a character progresses from SS2 to SS3 in canon, it may or may not be possible for Vegeta's preincarnation to progress to SS3. (For example, if the progression relies on outside factors or elements not present in STE.)