It would be useful to have some more hints about effects visible on enemies or how they react to some types of attacks. I mean things are characters could notice during combat.
[ COMBAT SPOILER AHEAD ]
I play 5e and suffered a total-party-wipe from a vampire who did not need the help of his two spawns, so I had a clue going in of how fearsome vampires are in 5e.
Especially as the previous female vampire I met in Solasta drained and killed my paladin in one single hit. I did Revivify on him and my paladin continued on with 8 HP maximum (+5 from Aid) and he still tanked like a boss. :P
I tried against Mardracht for fun with an exhausted party with low HP: failed.
Did a full rest, buffed, and wiped the floor with them (refusing the non violent option against undead). My paladin went ham on the vampire (and I did suffer a lot of damage back), along with my wizard, while the two others managed the undead bodyguards.
Tactical Adventures made Mardracht an old, insane and decrepit vampire --read: much weaker on some aspects-- but it is hard to say because I cannot tell if he was regenerating or not (does not seem so). He mostly did a lot of damage with basic spells cast at higher spell slots.
My main issue was waiting for him to no longer have Counterspell ... I waited a bit and later successfully cast Fireball.
NB :
I tried Daylight for fun, as the description in Solasta says "Summons daylight for you to control" :P, but realized after the fight that it is just a big bright light and should not affect vampires. On the bright side, it was a clean fight with no disadvantages on attack.
In comparison, the seemingly younger female vampire was more fearsome with her draining effect of doom !