Same issue here.
A manual install seemed to allow me to update to 2.0.
See the GitHub -
https://github.com/de-vnull/vnstat-on-merlin
--
/usr/sbin/curl --retry 3 "https://raw.githubusercontent.com/de-vnull/vnstat-on-merlin/main/dn-vnstat.sh" -o "/jffs/scripts/dn-vnstat" && chmod 0755...