diff --git a/lib/vagrant/environment.rb b/lib/vagrant/environment.rb
index f0b8114ff..65786e3eb 100644
--- a/lib/vagrant/environment.rb
+++ b/lib/vagrant/environment.rb
@@ -955,7 +955,7 @@ module Vagrant
       provider = guess_provider
       vagrantfile.machine_names.each do |mname|
         ldp = @local_data_path.join("machines/#{mname}/#{provider}") if @local_data_path
-        plugins << vagrantfile.machine_config(mname, guess_provider, boxes, ldp)[:config]
+        plugins << vagrantfile.machine_config(mname, guess_provider, boxes, ldp, false)[:config]
       end
       result = plugins.reverse.inject(Vagrant::Util::HashWithIndifferentAccess.new) do |memo, val|
         Vagrant::Util::DeepMerge.deep_merge(memo, val.vagrant.plugins)
diff --git a/lib/vagrant/vagrantfile.rb b/lib/vagrant/vagrantfile.rb
index 27d9dcb52..3dc7ada99 100644
--- a/lib/vagrant/vagrantfile.rb
+++ b/lib/vagrant/vagrantfile.rb
@@ -112,7 +112,7 @@ module Vagrant
     # @param [Pathname] data_path Machine data path
     # @return [Hash<Symbol, Object>] Various configuration parameters for a
     #   machine. See the main documentation body for more info.
-    def machine_config(name, provider, boxes, data_path=nil)
+    def machine_config(name, provider, boxes, data_path=nil, validate_provider=true)
       keys = @keys.dup
 
       sub_machine = @config.vm.defined_vms[name]
@@ -127,7 +127,7 @@ module Vagrant
       box_formats      = nil
       if provider != nil
         provider_plugin  = Vagrant.plugin("2").manager.providers[provider]
-        if !provider_plugin
+        if !provider_plugin && validate_provider
           providers  = Vagrant.plugin("2").manager.providers.to_hash.keys
           if providers
             providers_str = providers.join(', ')
@@ -145,18 +145,22 @@ module Vagrant
             machine: name, provider: provider, providers: providers_str
         end
 
-        provider_cls     = provider_plugin[0]
-        provider_options = provider_plugin[1]
-        box_formats      = provider_options[:box_format] || provider
+        if validate_provider
+          provider_cls     = provider_plugin[0]
+          provider_options = provider_plugin[1]
+          box_formats      = provider_options[:box_format] || provider
 
-        # Test if the provider is usable or not
-        begin
-          provider_cls.usable?(true)
-        rescue Errors::VagrantError => e
-          raise Errors::ProviderNotUsable,
-            machine: name.to_s,
-            provider: provider.to_s,
-            message: e.to_s
+          # Test if the provider is usable or not
+          begin
+            provider_cls.usable?(true)
+          rescue Errors::VagrantError => e
+            raise Errors::ProviderNotUsable,
+              machine: name.to_s,
+              provider: provider.to_s,
+              message: e.to_s
+          end
+        else
+          box_formats = provider
         end
       end
 
diff --git a/test/unit/vagrant/vagrantfile_test.rb b/test/unit/vagrant/vagrantfile_test.rb
index 129bb5874..d4b99445f 100644
--- a/test/unit/vagrant/vagrantfile_test.rb
+++ b/test/unit/vagrant/vagrantfile_test.rb
@@ -368,6 +368,38 @@ describe Vagrant::Vagrantfile do
         to raise_error(Vagrant::Errors::ProviderNotUsable)
     end
 
+    context "when provider validation is ignored" do
+      before do
+        configure do |config|
+          config.vm.box = "base"
+          config.vm.box_version = "1.0"
+          config.vm.define :guest1
+          config.vm.define :guest2
+
+          config.vm.provider "custom" do |_, c|
+            c.ssh.port = 123
+          end
+        end
+
+        iso_env.box3("base", "1.0", :custom, vagrantfile: <<-VF)
+        Vagrant.configure("2") do |config|
+          config.vagrant.plugins = "vagrant-custom"
+        end
+        VF
+      end
+
+      it "should not raise an error if provider is not found" do
+        expect { subject.machine_config(:guest1, :custom, boxes, nil, false) }.
+          not_to raise_error
+      end
+
+      it "should return configuration from box Vagrantfile" do
+        config = subject.machine_config(:guest1, :custom, boxes, nil, false)[:config]
+        expect(config.vagrant.plugins).to be_a(Hash)
+        expect(config.vagrant.plugins.keys).to include("vagrant-custom")
+      end
+    end
+
     context "local box metadata file" do
       let(:data_path) { double(:data_path) }
       let(:meta_file) { double(:meta_file) }