It doesn't really sound like it. Should we keep the weretiger curse ability?
Sure, it makes them distinctive.
So, are we going to make these a template à la the SRD vampire or a standard monster like the Spectre?
I'll vote for standard monster, if only for simplicity.
Do they need to turn into mist to fly? I'd prefer it if they could fly when in corporeal form, and the "or" in "turn into mist or fly" suggests they can do this.
Shall we start on the stats.
Medium Undead is pretty obvious, as is 10 Hit Dice.
40 ft. ground and fly speed?
Let's talk about ability scores? Here are some baselines:
Spectre: Str --, Dex 16, Con -, Int 14, Wis 14, Cha 15
Vampire: Str 17, Dex 14, Con -, Int 13, Wis 13, Cha 14
(standard array plus racial Str +6, Dex +4, Con -, Int +2, Wis +2, Cha +4)
Vampire Spawn: Str 16, Dex 14, Con -, Int 13, Wis 13, Cha 14
As 'super-vampires' they probably have higher stats than a standard Vampire or Spectre. We could giv them whichever stat is higher, with an additional +2 on top. That would work out:
Vampire-Spectre: Str 19, Dex 18, Con -, Int 16, Wis 16, Cha 17
Does that look OK, or is it too much?