Here's how the LA would work in your progression W0LF. Note at even levesl there's no racial HD, BAB, SAVES, or SKILLS. Those are the total of 6 LA from the combined Half Fiend LA +4 and the Half-Vampire LA +2 from Libris Mortis (pg 107~108). On those levels you'd get what your class provides on the other side of the gestalt. I didn't alter how you split out any of the class specials at all, simply added the LA into the progression evenly.
Monster Class Progression
| Level | Hit Dice | BAB | Saves | Skill Points | Ability Adj | Special |
| 1 | as class | as class | as class | as class | Str +2, Int +2 | NA +1, 2 Claws, Darkvision 60ft, Outsider Type, Spell-like Abilities, Blood Drain (lesser), Bonus Feats (Alertness, Dodge) |
| 2 | as class | as class | as class | as class | Dex +2 | Resistances (Cold/Elec 5), Spider Climb, Bonus Feat (Improved Initiative, Lightning Reflexes), Dhampir Skills +2 |
| 3 | as class | as class | as class | as class | Con +2, Cha +2 | Bite, Poison Immunity, HF Resistances (5), Spell Res (Level), Slam Attack |
| 4 | as class | as class | as class | as class | Str +2 | Blood Drain (Greater), Resistances (Cold/Elec 10), Dominate (lesser), NA+1, Dhampir Skills +4 |
| 5 | as class | as class | as class | as class | Dex +2, Int +2 | Children of the Night (lesser), Damage Reduction 5/Magic, |
| 6 | as class | as class | as class | as class | Con +2 | Dominate (moderate), NA +1 |
| 7 | as class | as class | as class | as class | Str +2, Cha +2 | Fast Healing 1/rnd, Children of the Night (greater) |
| 8 | as class | as class | as class | as class | Dex +2 | Dominate (superior), NA +1 |
| 9 | as class | as class | as class | as class | Con +2, Int +2 | Damage Reduction, Smite Good, Dominate (greater) |
| 10 | as class | as class | as class | as class | Str +2 | Dhampir Skills +6 |
| 11 | as class | as class | as class | as class | Dex +2, Cha +2 | Damage Reduction 10/Magic, NA +1 |
| 12 | as class | as class | as class | as class | Wis +2 | HF Resistances (10), Spell Res (+10), Wings |
Edit: If you're attempting to use the Vampire Spawn from the Monster Manual 1 I don't think it ever got a Level Adjustment published for it. As far as I'm aware Libris Mortis is the only place they published a viable half-vampire with a listed LA but that doesn't sound like its what you really wanted to make. I could certainly be wrong though...
Edit 2: I've adjusted it as appropriate. Since you have an LA of +12 there are no racial HD to account for. That means its like a normal character with all the standard class progression stuff, it just gets all the special stuff added as appropriate at each level.