In the Roll20 electronic version of Curse of Strahd, Strahd's stat block has been updated at least twice, in patches 1.9 (June 2020) and 1.3.
The current version (June 2020) reads:
Unarmed Strike (Vampire or Wolf Form Only)
Melee Weapon Attack: +9, Reach 5 ft, one target
Hit: (1d8+4) slashing damage plus 14 (4d6) necrotic damage
If the target is a creature, Strahd can grapple it (escape DC18) instead of dealing the bludgeoning damage.
This appears to me to follow the most recent revision of CoS, 6th Printing, published 06/17/2020, available at https://www.dndbeyond.com/forums/dungeons-dragons-discussion/rules-game-mechanics/8760-official-wizards-of-the-coast-errata#c16.
That this version matches yours, and is different from the earlier print version of CoS shown in Thomas Markov's answer, may mean that yours is actually the currently most correct version, but does little to clarify whether Strahd should be doing slashing or bludgeoning damage. However, I think it is safe to say that the "instead of" indicates that Strahd's grapple should do necrotic damage, only, regardless of what his unarmed strike does.
For reference, the current version of the Vampire statblock reads:
Unarmed Strike (Vampire Form Only): Melee Weapon Attack: +9 to hit, reach 5 ft., one creature.
Hit: 8 (1d8 + 4) bludgeoning damage. Instead of dealing damage, the vampire can grapple the target (escape DC 18).
(The 5th Edition SRD and the Vampire stat block within CoS differ only in some punctuation.)
The current version of Vampire Spawn reads:
Claws: Melee Weapon Attack: +6 to hit, reach 5 ft., one creature.
Hit: 8 (2d4 + 3) slashing damage. Instead of dealing damage, the Vampire can grapple the target (escape DC 13).
(The 5th Edition SRD and the Vampire Spawn stat block within CoS differ only in some punctuation)