This is solved according to this comment: https://github.com/varnish/newrelic_varnish_plugin/issues/13#issuecomment-26779670.
Basically we need to change the "default"
string in following line to something unique:
agent_human_labels("Varnish") { "#{@name||"varnish"}[#{@vname||"default"}]"}
Hope it will be patched soon to allow using other variable than vname.