Vargheists are a good fit with Mannfred, thematically. As units their relatively well balanced with each other, and their points costs under the new system are the same (160 per 3).
With their ability to inflict mortal wounds, albeit unreliably, flayers are ever-so-slightly more useful against enemy units with very good saves, particularly those not reduced by rend (bastilodon, spirit hosts, etc). Their scream is mostly negligible on its own, but if you play against low bravery enemies (humans, skaven, etc) AND you field other bravery-debuffing options like screaming skull catapults & the mournghul, then it may give flayers the edge there, as well.
In all other cases, including against most semi-elite units, vargheists will win out thanks to having damage 2 on their melee attacks.
A trio of vargheists is a reasonable addition to most undead forces if you just need something a little fast & hard hitting to top combats that are bogging down, or scoot down a flank to threaten rear units.