Probably quite low level. It has more HD than vampire spawn, if I read that correctly, but it should be quite a bit less powerful.
Copying from the vampire spawn, we have something like:
Medium Undead
Hit Dice: 5d12+3 (35 hp)
Initiative: +6
Speed: 30 ft. (6 squares)
Armor Class: 15 (+2 Dex, +3 natural), touch 12, flat-footed 13
Base Attack/Grapple: +2/+5
Attack: Slam +5 melee (1d6+4 plus energy drain)
Full Attack: Slam +5 melee (1d6+4)
Space/Reach: 5 ft./5 ft.
Special Attacks: Blood drain
Special Qualities: +2 turn resistance, damage reduction 5/silver, darkvision 60 ft., resistance to cold 10 and electricity 10, undead traits
Saves: Fort +1, Ref +5, Will +5
Abilities: Str 16, Dex 14, Con —, Int 13, Wis 13, Cha 14
Skills: Bluff +6, Climb +8, Craft or Profession (any one) +4, Diplomacy +4, Hide +10, Jump +8, Listen +11, Move Silently +10, Search +8, Sense Motive +11, Spot +11 (+5 ranks)
Feats: AlertnessB, Improved InitiativeB, Lightning ReflexesB, Skill Focus (selected Craft or Profession skill), Toughness
Environment: Any
Organization: Solitary or pack (2–5)
Challenge Rating: X
Treasure: Standard
Alignment: Always evil (any)
Advancement: —
Level Adjustment: —
If we're aiming for lower level, I say we drop the resistances (at least to electricity) and remove or reduce the DR. We may also want to reduce some of the abilities (which could affect the skill rank calculation). I'd probably give them a racial bonus on Disguise checks to appear as living humanoids, and we could give them Imp Grab to make the blood drain a little easier for them. If we make them a different type of vampire (rather than just a weaker vamp), we could justify some advancement, too.