So, did you want to cut the die size or number of dice for the poison breath, or both?
How's this? I'm leaning toward 1d4 rounds for the blinding, since that's close to the original's 2 rounds, but it does feel possibly weak.
Breath Weapon (Su): Ahi has three breath weapons: a cone of poison, a cone of scalding vapor, and a cone vaporizing breath.
The poison breath does 10d8 acid damage (DC X Ref save for half damage) and exposes any creature damaged to Ahi's poison.
The scalding breath does 20d8 fire damage (DC X Ref save for half damage), and any creature damaged must make a DC X Fort save or be blinded for 1d4 rounds.
The vaporizing breath does 10d8 nonlethal damage (DC X Ref save for half damage). If a creature is damaged by this breath, it must make a DC X Fort save, or it and any objects it carries turn into vapor as if they were affected by the gaseous form spell. This vaporous condition lasts for X minutes. Normally, the vaporous creature or object returns to solid form at the end of this duration, but if the vaporizing breath weapon did enough damage to destroy the creature or object it will dissipate into gas and a few puffs of dust when the duration expires. The traces of dust are enough to restore a dissipated victim with a resurrection spell. The vaporization effect can be neutralized by a dispel magic, remove curse, or break enchantment effect that succeeds at a caster level check against a DC equal to the Ahi's spell resistance. A vaporized victim can also be restored by any effect that can transform a gaseous object into a solid one, such as the polymorph any object spell or a vampire's gaseous form ability.
The save DCs are Constitution-based.
Poison (Ex): injury, Fort DC X, primary damage 2d6 Con, secondary damage 2d6 Con and unconsciousness. The Fort DC is Constitution-based.
That looks basically fine to me.
If you think the blinding duration is too short I have no objection to making it longer, although it might not make much practical difference considering how few melee round a typical high-level 3E combat lasts. Maybe 2d4 rounds?
The only other number we need to decide on is the duration of the gaseous form. The original was 5 to 10 melee rounds, or 1d6+4 minutes. I'd be fine with using that.