Cropping VR video afterwards
Using the mask image below, you can use ffmpeg to crop and output a MP4 with the visual content bounded in the same elliptical shape as the mask
ffmpeg -i input.mp4 -loop 1 -i 360mask.png \
       -filter_complex &quot;[0]split[a][b]; \
        [a]crop=iw/2:1522:0:300,crop=iw*0.8:ih*0.8,scale=960:-1,pad=960:960:0:150[l]; \
        [b]crop=iw/2:1522:iw/2:300,crop=iw*0.8:ih*0.8,scale=960:-1,pad=960:960:0:150[r]; \
        [l][r]hstack[c]; [c][1]alphamerge[ca]; [1][ca]overlay=shortest=1&quot;    vr.mp4

The video is split into two cropped halves to a rectangle which exactly bounds the visual area, then cropped to 80% along each of its dimensions, and then scaled and padded to a 960x960 canvas, rejoined to form a 1920x960 frame. Then the mask is applied to form the elliptical cut-out.
Mask


This is the basic technique. The visuals in your supplied image aren't centered, so I customized the crop expressions based on measurements within an image editor.