Vow of Dedication -
Requirement: Not already multiclassed
Con: Cannot Multiclass
Pro: At level 20, as part of the Perfect Self ability, the monk gains access to other vow's "Greater Boon." Bonus Feat: Defensive Roll (For epic dodge, basically)
Greater Boon: If the Monk has no other vows but this one, their Damage Reduction becomes 20/+3.
Vow of the Fist -
Con: All weapon/armor proficiencies are lost, and can never be taken via feats.
Pro: Unarmed attacks deal Piercing, Slashing, and Bludgeoning damage types.
Greater Boon: Ki Strike is 2 enhancement bonuses more powerful (+5 at 20, more later).
Vow of the Pure Body -
Requirement: Monk 12, Diamond Soul
Con: No Magic (No bound spells on items, no UMD, etc)
Pro: Diamond Soul grants SR = 15 + level
Greater Boon: SR = 20 + level
Vow of Clear Mind -
Con: No Alcohol (Perhaps also no potions), Cannot Take Drunken Master
Pro: Bonus on will and fort saves.
Greater Boon: Bonus increases.
Drunken Master -
Requirement: Level 10 monk, Diamond Body (Alcohol Poisoning immunity)
Con: Cannot take Clear Mind, Lose Empty Body
Pro: Drinking alcohol gives you bonuses to Str and minuses to Wis.
Greater Boon: Wisdom losses are halved.
Vow of Neutrality -
Requirement: Lawful Neutral
Con: Can never become a citizen/leader of a settlement.
Pro: Permanent protection from good and protection from evil
Greater Boon: Permanent aura versus good and aura vs. evil
Vow of Poverty -
Con: Cannot equip anything more than an unenchanted robe. No Bank Account. No bound spell usage (no magic item use).
Pro: - 2-5 ECL
Greater Boon: The ability to scry and/or teleport (similar to abundant step), however those mechanics work here.
Vow of Chains -
Con: Lose monk speed
Pro: Immunity to Slow, Woodland Stride
Greater Boon: Freedom of Movement
Vow of Brotherhood
Requirement: Level 15
CON: Lose Quivering Palm, Perfect Self does not grant DR 20/+1
PRO: Gain ability understand all languages.
Greater Boon: Gain the ability to speak all language (As Tongue of the Sun and Moon).
Vow of the Faithless
Con: Cannot choose a diety.
Pro: Nearby (Hostile?) Clerics have a spell failure chance (10% per 10 monk levels?). Alternatively, the Monk gains a bonus to saves vs. divine spells.
Greater Boon: Nearby Faithful characters cannot be aided directly by their gods (I'm not entirely certain how your deity system works, so this would need a hand!).
Vow of Honourable Combat -
Con: Lose/Cannot Take KD, IKD, Stunning Fist, Quivering Palm, Called Shot, Disarm, Improved Disarm
Pro: Bonus to discipline at +1 per monk level
Greater Boon: Bonus Doubles (effective immunity)
Vow of Exposure/Truth -
Con: -2 to bluff, hide, move silently per level
Pro: +2 to to spot, listen. Perma see invis.
Greater Boon: Perma True Seeing
Vow of the Martyr
Con: Lose Wholeness of Body
Pro: Gain Lay on Hands
Greater Boon: On your death, surrounding allies are effected by a Greater Restoration spell. Surrounding dead allies are effected by a Resurrection spell. Your corpse is destroyed, and cannot be raised.