The way I did it this time (the first time I had any problems with this quest):
1) Was asked for evidence.
2) Used key to enter house, sneaking, used invis potion and successfully reached the room with evidence, picked the lock and took the evidence letter
3) Still sneaking used an invis potion again, and left without problems.
4) Presented evidence and was asked to go to Ashfallow Citadel.
5) Killed a lot of Morag Tong assassins, but at the end it became a bit chaotic with Morag Tong rushing out from the last room. They got hurt badly by their own swinging traps there, and Vendil Severin escaped.
6) A bit unusual for this game, Vendil run towards Raven Rock, I could it see on the map, but could not reach him in time.
7) I caught him in his house and killed him.
8) Quest concluded, house received - all the items there were correctly transferred in ownership.
9) Trying to sleep or wait inside the house was not possible 'You cannot sleep when asked to leave'.
The body of Vendil remained, even after traveling back to Skyrim, and returning. It remained in the house for at least two in game days.
I did try to investigate, used the console, but always went back to a save before console usage:
+ I checked the location of Mirri and Tilisu Severin. They are both in the 'Death Room', where (named) NPCs end up after being killed. Ancano is also there for example.
+ The location of Vendil Severin was at the Severin house, obviously.
+ Disabling Vendils Body did not change anything, except him being not visible.
After a reading a bit about problems with this quest, I tried the following:
1) Using console I moved Vendils body to Ashfallow Citadel (where he would be, had I am killed there).
prid VendilsID
fast ravel to Ashfallow Citadel, entrance room and there
moveto player.
Vendils body appeared and lies there.
Fast raveling back and going into Severin Manor - sleeping and waiting is now possible. I accepted this fix and have no problems since.
The problem was created, because the NPC seems to flee with low health towards his house. In earlier games I always killed him in Ashfallow Citadel, and also did not sneak with the result of both Mirri and Tilisu killed in the house. This time all 3 were in Ashfallow Citadel and Vendil ran back to his home. This being the only difference, I suspect Vendils body did not clean up correctly.
Version Info:
Windows 10 PC,
Skyrim SE 1.6.1070 with USSEP 4.33 installed, only very few mods, none of them should cause this: The Paarthurnax Dilemma, IcepenPenguins Classic Worldmap, Immersive Citizens, ZombiesBegone, and Bonewolfnozombies
Issue Data
Issue #34940: Quest 'Served Cold': After solving the quest, cannot wait or sleep in Severin House