Have you tried other players ?
Try other player like MPC-HC in order to check if Plex is the problem.
What is your OS and your CPU/GPU ?
Do you have a discrete card installed or Intel iGPU only ?
Ah, yes I didn't pay attention to NUC.
So, you have a Broadwell Core i5-5250U with HD 6000 GPU.
Haven't noticed any weird decoding using my Haswell Core i7-4790/ HD 4600 and the same OS and drivers like you.
Everything is the same regarding DXVA.
Is it possible to upload somewhere one of those VC-1 encoded videos ?
After further research I've concluded that the problem lies with PLEX not using current version of ffmpeg.
Their copy of FFMPEG is old and prone to this issue.
They say they are working on upgrading ffmpeg but it will require a larger update to the whole package.
So it will take a while...which sucks.
Here's the posting