This is an interpretation question. The channel evil section of the tiefling archetype "Fiendish Vessel" states:
At 1st level, a fiendish vessel, rather than channeling positive or negative energy, instead channels the pure evil power of her fiendish patron. This ability is similar to channeling negative energy, but instead of healing undead and dealing damage to living creatures, this blast of evil energy automatically heals evil creatures and debilitates good creatures within its burst.
Channeling this evil causes a burst that affects all creatures in a 30-foot radius centered on the fiendish vessel. In the case of evil creatures, the amount of damage healed is equal to 1d4 points of damage and increases by 1d4 at every two levels beyond 1st (to a maximum of 10d4 at 19th level). Good creatures in the burst receive a Will saving throw to negate this damage. Good creatures that fail their saving throws are sickened for 1d4 rounds.
The first paragraph seems to make clear that this power allows the fiendish vessel to heal evil creates, but only to "debilitate" good creates (as opposed to "damage" them - the previous sentence specifically uses the term "damage" when refering to channeling positive and negative energy - I assume this difference is deliberate.
The second paragraph confuses the issue by stating good creatures receive a saving throw to "negate this damage" but then goes on to state that failing the saving throw results in being sickened for 1d4 rounds.
I think the words "negate this damage" refer to the sickening given the use of the term "debilitate" in the first paragraph, but its confusing as it could mean that good creatures take 1d4 damage per two levels AND suffer 1d4 rounds of sickening. However, if that were the case why didn't the first paragraph refer to "damage and debilitate" instead of just "debilitate".
any help/advice would be welcome.