If you want her to be good at social skills then you'll want a couple of traits swapping social skills from Cha to Int. If you get a couple of free traits use those, if not you need a feat on additional traits.
If you want to be dex focused then you want two feats, weapon finesse and deadly agility. If not you can save those and perhaps spend them on harbinger-specific feats, sin eater and grasp of darkness.
The third feat should be power attack or piranha strike IMO (assuming you didn't need that for additional traits).
Edit: with the free advanced study or discipline focus feat - if you spend that on advanced study (scarlet throne) then that lets you spend your class level 3 maneuvers on defences from shattered mirror or veiled moon, if you spend it on discipline focus then your save DC for the dazing strikes etc of scarlet throne becomes OK but not great. Either works, just depends what you want.
Well, that's the question. Do you want to have 20 Dex, 18 Int and a scattering of lowish ability scores, or would you like 16 Str and a whole lot of 14s elsewhere? Both work but they're different characters.
The Crimson Countess doesn't get dark focus, so you don't have any greater incentive to specialize in a particular discipline. With 32 you could buy:
Str 8
Dex 16
Con 15
Int 16
Wis 12
Cha 14
(The last two can be switched depending if you want better Will and Sense Motive or more flexible social skills)
With dhampir adjustment and putting the lvl 4 stat boost into Con that gives you:
8
18
14
18
12
14
Student of Philosophy is probably the best trait for social skills, let's you use Int for Diplomacy to persuade and Bluff to lie. Bruising Intellect is also good and lets you use Int for Intimidate, but they're both social traits and the first sounds like a better fit.
I view Grasp of Darkness as pretty much mandatory if you want to use boosts or counters, would suggest:
1 Weapon Finesse
3 Deadly Agility
5 Grasp of Darkness
For maneuvers I'd view top picks as:
Circular Stance (1-ST) - cheap stance remains awesome against sneak attackers
Fading Strike (2-VM) - excellent maneuverability boost, worth picking up even if it's your only veiled moon maneuver
Garnet Lance (2-ST)- straightforward ignore DR
Sanguine Barrier (2-ST) - Needs Sense motive but lets you fully negate an attack
Word of Retribution (2-CR) - makes you feel okay about getting crit by the boss, needs curse
Dogpile Strike (3-CR) - awesome if you have melee allies, decent without, doesn't need curse
Scarlet Eye's Perception (3-ST) - when you need to hit something
Cunning liar (regional trait: any city) and cunning wordplay (social) are the traits I'd take. Student of philosophy would keep tripping over situations where it doesn't quite apply IMO.
Aldrakan's right, a crimson countess loses dark focus, ignore the last paragraph of my last post.
I should ask whether variant dhampir heritages are possible - a str-based build becomes much easier with those.
I think any of the dhampir subraces can get that feature, by the wording of the spell-like ability feature which heir replaces. The subraces don't replace that feature or even alter it, their spell-like ability is one of the options.
So, a jiang-shi born gets +2 Str & Int, -2 Dex. This is obviously better for a str-based character, which in turn has an easier time with the feats you'll want (no weapon finesse/deadly agility required means 2 more free feats).
Maybe Str 16+2 race=18, Dex 14-2 race=12, Con 16, Int 15+2 race+1 level=18, Wis 12, Cha 8, then take sin eater to get temp HP (which reduces your need for healing; important for dhampir), grasp of darkness to make maneuver recovery easier and to claim multiple targets, then either power attack or additional traits depending on whether you get 2 free traits. If you'd rather have some Cha for the image then drop Con to 14 and raise Cha to 13.
Aldrakan's dex/int build works well enough if you'd rather go that way.
Veiled Moon IMO. Scarlet throne is good for hitting people as is cursed razor; veiled moon is less good at hitting people but includes short teleports, combat illusions and counters to the same. You get a better range of effects by selecting two different disciplines than two similar ones.
Edit: for example you might get
Dazing strike (ST 3) - Fort save or be dazed & ~ +7 damage
Sanguine barrier (ST 2) - sense motive to negate an attack
Fading strike (VM 2) - teleport and attack, or attack and teleport, as a standard action
Inner sense (VM 1) - +2 to a save
Disturbing blow (VM 1) - attack and debuff enemies' next d20 roll
Scything strike (ST 1) - cleave
Blade of breaking (ST 1) - disarm or sunder
(only 5 of the above readied at a time; I'd start from the top unless you or your party have a plan)
stances:
Formless dance (VM 3) - see invisibility and blur
Scarlet einhander (ST 1) - +2 attack and +1d8 damage
Aura of misfortune (Cursed razor 1) - -2 to enemies' saves within 35'