Don't provide a default for the environment. That way currently the environment that is currently set for a node on the server is not mistakenly overridden with "_default".

lib/vagrant/provisioners/chef_server.rb

@@ -23,7 +23,6 @@ module Vagrant
           @client_key_path = "/etc/chef/client.pem"
           @file_cache_path = "/srv/chef/file_store"
           @file_backup_path = "/srv/chef/cache"
-          @environment = "_default"
         end
 
         def validate(errors)

templates/chef_server_client.erb

@@ -13,7 +13,7 @@ client_key             "<%= client_key %>"
 file_cache_path    "<%= file_cache_path %>"
 file_backup_path   "<%= file_backup_path %>"
 
-<% unless environment == "_default" %>
+<% unless environment.nil? %>
 environment        "<%= environment %>"
 <% end %>