Well, I compiled new Cingg git (gcc 5.5.0,  ~ Slackware 14.2,  32 bit (!)) 

file 1920x1080 24 fps h264 mkv from youtube

no plugins/effects/transitions

default project format - rgba-8bit

and va-api works for me on both decoding (most visible cpu usage reduction if i set output to x11-direct - 60% g> 30%), on opengl output it really clise to same 60-70% cpu utilization (one core, dynamic freq.) and encoding

you need to restart Cin after changing hw accel method

But biggest thing was va-api h264 encoding  - 130% cpu and 18 fps vs nearly 400% cpu and 2 fps (while file size more like 5 mb for software vs 13 mb for vaapi). And memory usage with sw encoding much higher = 800/900 mb vs 300 mb in va-api case. I nearly deadlocked this 4gb ram laptop while trying ram-only (tmpfs) build and encoding tests... zram + zswap  helped surely...