You need to include the module to use the class without ::s.

plugins/guests/arch/cap/configure_networks.rb

@@ -6,6 +6,8 @@ module VagrantPlugins
   module GuestArch
     module Cap
       class ConfigureNetworks
+        include Vagrant::Util
+
         def self.configure_networks(machine, networks)
           networks.each do |network|
             entry = TemplateRenderer.render("guests/arch/network_#{network[:type]}",