>     disklabel -r -w vn0s0 auto
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>
>this gives me error saying no such file or directory

You are missing /dev/vn0 device nodes, make these by:
     cd /dev
     sh MAKEDEV all    # or just 'sh MAKEDEV vn0'

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>And also in the man page there is this comment for the following command.
>Can you tell me what exactly i have to edit in the file.
>  disklabel -e vn0s0      # edit the label to create a vn0s0a partition

Simplest is to just make the whole disk one partition:
duplicate existing 'c:' partition line at bottom and edit 1st copy: 'c:' -> 'a:', 'unused' -> '4.2BSD'.
Details are in disklabel(8) (man disklabel).

Hope his helps.

 -thomas