Freedoom1.wad should be fully vanilla with these maps. I know people are already updating or replacing some, but this is just to get it done...right now. Any replacement or updated maps can replace my vanilla fixes. 

I focused on removing all visplane overflows using visplane explorer, with open doors setting. Also drastically reduced drawseg overflows in maps that also had VPO. VPOs in areas a vanilla player cannot possibly reach are ignored. Drawseg overflows in small distant areas or windows are also ignored. 

All/most maps tested to exit using -nomonsters or at least I walked around areas I changed.

E1M1 - Fixed massive drawseg overflow on cliff, with doors closed. Only a minor coop issue now. 
E1M5 - Railings in crate room simplified. Some tweaks in main area near start, mostly visible through doors. Blue key area simplified a good bit. Lighting fixtures in various places simplified. Removed 2 doors at start.
E2M2 - Simplified and squared many light fixtures and details.
E2M4 - Small VPO in long hallway, removed a border.
E2M6 - Drawsegs, light pillars and stairs.
E2M9 - Simplify star near end, undetail areas near start.

E3M7 - Move around some vertexes in the secret.
E3M9 - Extremely light touch, just combined a few stairs. 
E4M2 - Lowered and joined cieling sectors right next to start. 
E4M7 - Ungradient light in one corner. 

MAP02 - Drawsegs not nearly as glaring, no VPO. Mostly light gradients and a few details. 
MAP03 - Remove a couple light gradients. 
MAP05 - Remove floor crack in lava area. Tiny VPO
MAP08 - Simplified ceiling lights and gradients. 
MAP09 - Simplify small redrock area.
MAP10 - Mostly trim light gradients.
MAP11 - Light fixtures and some detail.

MAP12 - Everything. Still far from finished, but its vanilla now. Gameplay is nearly untouched. I did not focus on texture alignment or good selection, will need lots of work. Lots of HOM, most of it is bearable. Kept railroad tracks and the most basic of factory details. There is tons of very long sight paths, if someone wants to remake areas so you cant see entirely through map. Details can be readded if areas are redesigned without long sight paths. This is vanilla and is a perfect base for someone to redetail and touch up. Playtested with nomonsters to exit, checking on trouble spots. The single archvile that can jump you isnt high enough to VPO...that I know of. Linking resources at bottom of post for research.

MAP18 - Tiny VPO, modified sky area in corner.
MAP20 - Removed one of the three little rooms with fancy effects after yellow key door.
MAP22 - Remove a good bit of ceiling detail from last room, a few other nice details.
MAP25 - Simlify a few things near start. Adjust 3 key room.
MAP29 - Lots of simplified geometry. Many pillars at start created solidseg overflows, remade into larger but fewer pillars. MAP29 backups in link at bottom of post include VPO free versions with intact pillars...mine are kinda bad, if someone wants to remake them.