I play daemons as well as vampires and this is a matchup that isnt great for any of the armies but usually worse for vampires than the other way around.
Important things to consider
flamers - have moved to special, increased cost, gets penalty for multiple shots, S4 instead of S5 in combat, may give the target unit D3 extrawounds or 6+ regeneration save. Taken together a lot of daemon players will put them on the shelf or at least run them in smaller units such as 2 x 3 flamers.
Screamers - slashing attacks now need to roll to hit, they do D3 wounds on large targets. Overall they get better and instead of not being included they will now be present in many lists.
New chariots should be good at killing lots of weak infantry with impact hits and various other stuff.
They can really hurt your LD if they want to. -2 LD banner, access to lore of death, access to lore of slaanesh, masque special character etc
They have access to lore master ability also for hero level casters. Most popular lore for daemons around here is lore of light that gives amazing buffs to many daemon units. Personally I dont use thirster but a lvl4 caster with lore of light. Twin heads gives an additional +2 to cast. Of course lore of light also has the ability to really hurt vampires and especially if they have more than one caster that knows the lore (+1S for each caster to spell that does 4D6 hits on undead)
They have some really naste special characters. Worst one is probably Kairos. Each head picks spells from any of the lores in the rule book. This means that we will be able to cast throne of wines, dwellers from below, purple sun and some crazy combos of buffs using the same caster with a +6 to cast modifier.
Changeling, swaps stats with anyone in base to base contact
Lots of stuff including GD fly or run very fast M10 such as M10 fiends.
Access to multiple dispel scrolls
Importantly their main weakness is that they have a very low number of units and they cant to all of the above at the same time. Pick your combats and avoid the really nasty stuff.
Also all daemon core except for plaguebearers are rather expensive models with T3 and no armour save (5+ ward) meaning that VC core can actually get kills and potentially win through attrition. Of course buffs or curses from any side can change everything.