I'm not too sure about Xnviews default mode "QTAV", but I believe you need ffmpeg.exe inside the addon folder.
I use codecs aka direct show instead. You need to change videomode=false to =true, under general section of the .ini file.
If still no thumbs, that means your missing codecs, like k-lite. just install these and restart your pc.
also sometimes changing .exe association back and forth fixes this too.
To open in vlc you need to set options in filelist>> custom filter. or right click open in.
Windows 7 Ultimate, x64-bit (SP 1) XnViewMP x64 v0.85-b-1 Port
i5-3570K CPU @ 3.40GHz
Memory: x16.0 GB
Samsung SSD 840 EVO 250GB ATA
NVIDIA GeForce GTX 960 @ 1177 MHz
Dedicated: 4096 MB GDDR5