This is a really long post.
I'll try putting together a comparison model. Human Monk 20 Vampire 8/Monk 12. SRD stuff only to keep the choices simpler and I'm not bothering with equipment or listing skill choices. Idental works otherwise. I've steered several choices to favor the Vampire, such as only giving the characters 10 Con. Neither is meant to be particularly min-maxed.
Okesh
Male Human Monk 20
Lawful Good
Strength 18 (+4) (2 Level Bonus Points placed)
Dexterity 13 (+1) (1 Level Bonus Point placed)
Constitution 10 (+0)
Intelligence 10 (+0)
Wisdom 18 (+4) (2 Level Bonus Points placed)
Charisma 8 (-1)
Size: Medium
Total Hit Points: 95 (20d8, 8+4.5*19)
Speed: 90 feet [monk]
Armor Class: 19 = 10 +1 [dexterity] +4 [wisdom] +4 [monk level]
Touch AC: 19
Flat-footed: 18
Initiative modifier: +5 = +1 [dexterity] +4 [improved initiative]
Fortitude save: +12 = 12 [base]
Reflex save: +13 = 12 [base] +1 [dexterity]
Will save: +16 = 12 [base] +4 [wisdom]
Attack (handheld): +19/+14/+9 = 15 [base] +4 [strength]
Attack (unarmed): +20/+15/+10 = 15 [base] +4 [strength] +1[feat]
Flurry of Blows: +20/+20/+20/+15/+10 [includes strength modifier and feat]
Attack (missile): +16/+11/+6 = 15 [base] +1 [dexterity]
Grapple check: +19/+14/+9 = 15 [base] +4 [strength]
Unarmed Damage: 2d10 +4 [strength]
Feats:
Blind-fight
Improved Trip [monk]
Deflect Arrows [monk]
Improved Critical Unarmed
Improved Initiative
Improved Unarmed Strike [monk]
Stunning Fist [monk]
Leadership
Power Attack
Cleave
Run
Weapon Focus Unarmed
Special Abilities
Human:
* Extra feat at first level (already included)
* Four extra skill points at first level (already included)
* One extra skill point at each additional level (already included)
Monk:
* AC Bonus for Wisdom
* AC Bonus for level (begins level 5)
* Flurry of Blows
* Unarmed Strike
* Evasion (level 2)
* Fast Movement (already included)
* Bonus Feats (levels 1 2 & 6)
* Evasion (level 2)
* Fast Movement (level 3)
* Still Mind level 3)
* Ki Strike (level 4)
* Slow Fall (level 4)
* Purity of Body (level 5)
* Wholeness of Body (level 7)
* Improved Evasion (level 9)
* Diamond Body (level 11)
* Quivering Palm (level 15)
* Timeless Body (level 17)
* Tongue of Sun and Moon (level 17)
* Empty Body (level 19)
* Perfect Self (level 20)
Kesho, Okesh' evil twin
Male Vampire Human Monk 12
Lawful Evil
Strength 23 (+6)
Dexterity 16 (+3)
Constitution --- [undead]
Intelligence 12 (+1)
Wisdom 20 (+5)
Charisma 12 (+1)
Total Hit Points: 84 (12d12 12+6.5*11)
Speed: 70 feet [monk]
Armor Class: 26 = 10 +3 [dexterity] +5 [wisdom] +2 [monk level] +6 [vampire]
Touch AC: 20
Flat-footed: 23
Initiative modifier: +7 = +3 [dexterity] +4 [improved initiative]
Fortitude save: +8 = 8 [base]
Reflex save: +13 = 8 [base] +3 [dexterity] +2 [lightning reflexes]
Will save: +13 = 8 [base] +5 [wisdom]
Attack (handheld): +15/+10 = 9 [base] +6 [strength]
Attack (unarmed): +16/+11 = 9 [base] +6 [strength]
Flurry of Blows: +16/+16/+16/+11 [includes strength modifier]
Attack (missile): +12/+7 = 9 [base] +3 [dexterity]
Grapple check: +15/+10 = 9 [base] +6 [strength]
Feats:
Alertness
Improved Trip [monk]
Combat Reflexes
Dodge
Improved Critical Unarmed
Improved Initiative
Improved Unarmed Strike [monk]
Stunning Fist [monk]
Leadership
Lightning Reflexes
Power Attack
Cleave
Run
Weapon Focus Unarmed
Human:
* Extra feat at first level (already included)
* Four extra skill points at first level (already included)
* One extra skill point at each additional level (already included)
Vampire:
* Undead with d12; usual undead traits
* +6 strength, +4 dexterity, no constitution, +2 intelligence, +2 wisdom, +4 charisma
* Turn resistance +4
* +8 racial bonus on bluff, hide, listen, move silently, search, sense motive, and spot
* Electricity and cold resistance 10
* Damage reduction 10 / silver and magic
* Slam attack, energy drain; blood drain
* Gains alertness, combat reflexes, improved initiative, lightning reflexes; also dodge if Dexterity>=13
* More vampire abilities and limitations
Monk:
* AC Bonus for Wisdom
* AC Bonus for level (begins level 5)
* Flurry of Blows
* Unarmed Strike
* Evasion (level 2)
* Fast Movement (already included)
* Bonus Feats (levels 1 2 & 6)
* Evasion (level 2)
* Fast Movement (level 3)
* Still Mind level 3)
* Ki Strike (level 4)
* Slow Fall (level 4)
* Purity of Body (level 5)
* Wholeness of Body (level 7)
* Improved Evasion (level 9)
* Diamond Body (level 11)
Okay, now that that mess is out of the way let's see how our two monks compare at three common hazards of adventuring, dealing with a nasty incoming spell, protecting the party mage from an incoming barbarian, and infiltrating an enemy stronghold to kill the evil within.
Stage 1: 20th level Evil Wizard Vilebrood fires off a hostile spell against the monks. I'm maximizing the spells to remove a random element.
Vs. Distintigrate:
Vilebrood must succeed in a ranged touch attack. Kesho's Touch AC is 20, Okesh' is 19. This gives Kesho a slight advantage I'm not going to bother calculating because I can't be bugged to figure out a 20th level wizard's stats on top of these. So assume it hits.
Okesh's Diamond Soul has a 50% chance of resisting the spell entirely. He further has a Con save of +12 to reduce damage to 5d6 (30HP). Otherwise the maximized 40d6 will kill him.
The DC is 10+6+10 or 26.
Okesh will die on a roll of 13 or less, meaning he succeeds 35% of the time he has to roll. Since he only has to roll half the time this means that:
Okesh takes no damage 50% of the time.
Okesh dies 17.5% of the time.
Kesho has no Diamond Soul so he never resists the spell through SR. His Con Save is +8.
Kesho will die on a roll of 17 or less.
This means:
Kesho Takes no damage 0% of the time.
Kesho Takes 30 points of damage 15% of the time.
Kesho dies 85% of the time.
The other spells I looked at tend to have similar results, although both of them are tough enough to survive one maximized fireball.
Stage 2: Now the team (Having ressurected Kesho) is attacked by Vilebrood's minions. A lvl 15 Barbarian is charging the party's Wizard and only the Monk stands in his way. Let's assume the player chooses to trip the Barbarian, buying time to keep him down for a flurry.
The Barbarian is decked out for battle, with a touch AC of, let's call it 12. They only have one turn to do this or the Barbarian will get past them.
Both have high enough BAB's to succeed in the touch attack on anything but a 1, so assume they both succeed. Now for the opposed test.
Okesh' Modifier is +8 for strength and feat. The Barbarian gets +4 str and rolls a 10. Okesh can only fail on a roll of 5 or less, succeeding in his trip 75% of the time.
Kesho's modifier is +10 for strength and feat while the Barbarian is the same as before. Kesho can succeed on a roll of 4 or more, meaning he succeeds 85% of the time. Kesho is the clear winner this time although his margin of error is much smaller than the other test.
Stage 3:Now the Party reaches Vilebrood's Castle. It's time to sneak in through a window and save princess Brjost from Vilebrood.
Okesh can reliably count on a Move Silently of +24 for skill and ability. Kesho can do the same using his racial modifier of +8 to make up for his lost levels. Okesh gets a slight edge by using his Tongue of the Sun and Moon to ask some random plants and insects for their insights into the castle, gaining only minor relatively insignificant information, but getting to feel cool for his contribution. Then they both go in. . .
But wait! unfortunately Kesho's Vampiric nature prevents him from going in uninvited. He'll have to wait outside. Maybe if the GM is feeling generous the player can control Kesho's cohort Igor for the rest of the mission.
All in all my feeling is that while there's some few things the Vampire can do better than a straight core character, he's unlikely to survive long enough to get to try, and the horrible horrible vampiric weaknesses not only kill the vampire, they will tend to kill the game unless the GM carefully remembers to avoid sunlight, running water, any non-public dwelling (does that include your average dungeon?) etc.