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Yes if I am understanding your question you can do this easily without any app.

Just start your video in landscape mode (so the camera icon turns) then turn it vertically after it begins recording.
 
Yes if I am understanding your question you can do this easily without any app.

Just start your video in landscape mode (so the camera icon turns) then turn it vertically after it begins recording.
Tried it. Its landscape, but sideways.
 
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