When using its Unarmed Strike Action, a Vampire can grapple instead of dealing damage, with an escape of 18.
It also has Sunlight Hypersensitivity:
While in sunlight, it has disadvantage on attack rolls and ability checks.
With Passive Perception we know to substract 5 for disadvantage, but is this also the mechanic for escape DCs?
Best Answer
The DC does not decrease in sunlight
It looks like your hypothesis is that the DC of the grapple is just the passive athletics check of the vampire, and thus if the vampire is in sunlight, disadvantage would change that passive score, and thus the DC. However, we can prove that this isn't the case.
Based on page 297 of the Monster Manual, we know that a vampire has a +4 strength modifier. Additionally, its unarmed strike attack, which uses strength, has a +9 to hit. From this, we can establish that the vampire has a +5 proficiency bonus.
Vampires are not proficient in athletics, so we can determine that the passive athletics score of a vampire, which is what would be used to maintain a grapple, is 14, not 18. Furthermore, if the vampire was proficient with athletics, the passive roll would be 19, not 18. Since neither of these possibilities match the DC of the grapple, we can assume that the escape DC of the grapple is not based on a passive ability check, but is rather a static DC. Since static DCs are not necessarily ability checks, there is no need to apply the -5 to the DC for being in sunlight.