I had expected the NPCs to not be very close to their OG likenesses and am very surprised to see how on-model not only Martin, but also Lucien and Vincente look.

The main differences here appear to be:

Martin: his eyes are further apart closer to average. His eyebrows have the same overall shape despite the texture quality difference.

Lucien: slightly more obvious curve on the upper lip, and he has a more obvious beard now (similar to Martin, IMO this was the OG stubble texture being like 20px in resolution). Nose has an asymmetrical bump at the base.

Vicente: his head isn't as broad but keeps his overall silhouette, and his eyes are at less of an angle now - but they have changed colour, vampires appear to all have icy blue eyes in the remaster.

This close of a 1:1 match is likely so consistent across all the NPCs (see: the Adoring Fan and some of the wonkier ones) because Virtuos and Bethesda made new models for the character creation shape keys, and probably did not attempt to resculpt anyone to have unique faces. Most of the OG NPCs are made with the same CC that players use, and the same is true here.