VirtualDub - Video - Compression
There is an option in the Xvid settings : Profile @ Level, then click the button more and place a check mark next to the item Interlaced encoding.
Your terms: left - right have no meaning, I dare to say, to most of us; not without a sample. If you mean even fields and odd fields, then you can encode interlaced, even with Xvid, as shown above.
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