So out of curiosity I built 2 monks. I'm not the best at building optimized characters, but here's the stats (used PCGen for what it is worth, 28 point buy, 110000gp.):
Melee Monk, Male Dwarf Monk13 CR 13; HD (13d8)+39; hp 107; Init +4 (+4 Dex, +0 Misc); Spd Walk 60'; AC 26 (flatfooted 22, touch 21), SA: +4 Dodge bonus to Armor Class against monsters of the giant type, +1 racial bonus on attack rolls against orcs and goblinoids, +2 racial bonus on saving throws against poison, +2 racial bonus on saving throws against spells, Abundant Step (Su) ~ 1/day, AC Bonus (Ex), Diamond Body (Su) ~ Immune to poisons, Diamond Soul (Ex), Fast Movement (Ex), Flurry of Blows (Ex), Improved Evasion (Ex) ~ No damage on successful save, half damage on failed save, Ki Strike (Su) ~ (Magic, Lawful), Purity of Body (Ex) ~ Immune to natural diseases, Slow Fall (Ex) ~ 60 ft, Stability ~ +4 bonus on ability checks made to resist being bull rushed or tripped when standing on the ground, Still Mind (Ex) ~ +2 save vs. enchantment, Stonecunning ~ +2 racial bonus on Search checks to notice unusual stonework, can also intuit depth, sensing approximate depth underground, Stunning Fist 13/day (DC 18), Wholeness of Body (Su) ~ 26 hp/day; Vision: Darkvision (60'), Normal AL: LG; Sv: Fort +14, Ref +15, Will +13; STR 23, DEX 18, CON 16, INT 8, WIS 14, CHA 6
Skills and Feats: Jump +18; Cleave, Improved Bull Rush, Improved Grapple, Power Attack, Weapon Focus (Unarmed Strike)
Possessions: 1 Amulet of Mighty Fists +2, 1 Gloves of Dexterity +4, 1 Belt of Giant Strength +4, 1 Bracers of Armor +5, 1 Cloak of Resistance +3, 1 Ring of Protection +3
Innate: Dimension Door
Unarmed Attack: +18/+13 2d6+8
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Vampire Monk: Male Elf Monk5 ; CR 7; Medium RACETYPE ; HD (5d12); hp 40; Init +11; Spd Walk 40'; AC 35, touch 24, flat-footed 28, Base Atk +3; Grp+12; Atk: +14 (1d6+11/20/x2, *Slam ) or ; Full Atk: +14 (1d6+11/20/x2, *Slam ) or ; SA +2 racial saving throw bonus against enchantment spells or effects, AC Bonus (Ex), Alternate Form (Su) ~ can assume the shape of a bat, dire bat, wolf, or dire wolf as a standard action, An elf who merely passes within 5 feet of a secret or concealed door is entitled to a Search check to notice it as if she were actively looking for it, Blood Drain (Ex) ~ drains blood from a grabbed opponent dealing 1d4 point of Constitution damage each round it maintains a hold, Children of the Night (Su) ~ command the lesser creatures of the world and once per day can call forth 1d6+1 rat swarms, 1d4+1 bat swarms, or a pack of 3d6 wolves as a standard action, Create Spawn (Su) ~ A humanoid or monstrous humanoid slain by a vampire’s energy drain rises as a vampire spawn 1d4 days after burial, Dominate (Su) ~ crush an opponent’s will just by looking into their eyes (DC 11 Will save or fall instantly under the vampire’s influence), Energy Drain (Su) ~ creatures hit by a slam attack (or any other natural weapon) gain two negative levels, Evasion (Ex) ~ No damage instead of half damage on successful save, Fast Healing (Ex) ~ 5, Fast Movement (Ex), Flurry of Blows (Ex), Gaseous Form (Su) ~ as a standard action, can assume gaseous form at will as the spell (caster level 6th) but it can remain gaseous indefinitely and has a fly speed of 20 feet with perfect maneuverability, Immunity to magic sleep effects, Ki Strike (Su) ~ (Magic), Purity of Body (Ex) ~ Immune to natural diseases, Resistance to Cold (Ex) ~ 10, Resistance to Electricity (Ex) ~ 10, Slow Fall (Ex) ~ 20 ft, Spider Climb (Ex) ~ may climb on stone surfaces as though using the Spider Climb spell, Still Mind (Ex) ~ +2 save vs. enchantment, Stunning Fist 5/day (DC 15), Turn Resistance (Ex) ~ 4; SQ: Low-light, Normal; AL: LE; SV Fort +7, Ref +16, Will +10; STR 28, DEX 25, CON *, INT 11, WIS 16, CHA 12 STR 28, DEX 25, CON *, INT 11, WIS 16, CHA 12 .
Skills and Feats: Bluff +9, Hide +15, Jump +13, Listen +13, Move Silently +15, Search +8, Sense Motive +11, Spot +13; Alertness, Combat Reflexes, Dodge, Improved Initiative, Improved Unarmed Strike, Lightning Reflexes, Power Attack, Stunning Fist, Weapon Focus (Unarmed Strike).
Possessions: Amulet of Mighty Fists +2; Ring of Protection +3; Belt of Giant Strength +4; Bracers of Armor +5; Cloak of Resistance +3; Gloves of Dexterity +4; Slam;
Unarmed attack +15 1d8+11 and Energy Drain
So hmmm. Hitpoints less than half, but an AC 9 higher. Average about 14 damage slightly more often, or 15 damage and energy drain less often. Saves Except for fotitude its a wash, and as has been said he's immune to most of that fortitude save stuff. DR and energy effects favor the vampire. Differences in special abilities.
Seems viable to me, if played with a bit of caution.
-cpd