You probably installed a system wide Lagarith codec [maybe via some codec pack], the vdplugin thing comes with vdub2, and only works with vdub2.
EDIT:
This seens to be the Lagarith home site:-
https://lags.leetcode.net/codec.html
Quote:
Lagarith was last updated on December 8, 2011.
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Last official version v1.3.27.