So it worked initially?, what was it set to? DXVA2(native) or none ?
LAV settings can also be accessed from the start menu, K-Lite Codec Pack\Configuration
There will likely be 2 one for mpchc and one system-wide, set both to whatever worked, if settings crash, redo k-lite codec pack setup and configure those options during setup
Test dreamscene
Some applications may not respect LAV configuration settings, in that situation you will usually have to remove DXVA2-related dlls installed by the graphics driver so they will have no chance to use something broken, I'd done this once with AMD drivers
The fact that symptoms are the same between AMD and NVIDIA gpus however makes me think that you have a conflicting other application/driver installed as these are all old vista-supported gpus not something that needs custom drivers
EDIT: And did you uninstall AMD drivers with DDU before installing nvidia? (latest for vista - DDU v18.0.4.6.exe)