I have to convert files updloaded by users to both mp4 and web8. I'm running into problems with vertical (portrait mode) videos.

For example, Using ffprobe shows that my input.mp4 file is (1280*720, with a rotate 90), The video was shot vertically using android, and plays sideways in VLC, like the following diagram

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| o-< |
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So i applied the following transpose:
ffmpeg -i input.mp4 -vf transpose=1 intermediate.mp4

The resulting video plays vertically in VLC (it looks fine)
ffprobe shows (width:720, height:1280, aspect ratio 9:16)

If i had to take this intermediate.mp4 file and convert is into 640*360, to play in the web, what do i have to do?
I assume black bars need to be added to the side so that the video doesn't get stretched...how do i do this?