Environment#primary_machine takes a provider argument. Use it.

lib/vagrant/environment.rb

@@ -230,14 +230,15 @@ module Vagrant
     # method is only applicable for multi-VM environments. This can
     # potentially be nil if no primary VM is specified.
     #
+    # @param [Symbol] provider The provider to back the primary machine.
     # @return [VM]
-    def primary_machine
+    def primary_machine(provider)
       if machine_names.length == 1
-        return machine(machine_names[0], :virtualbox)
+        return machine(machine_names[0], provider)
       end
 
       config_global.vm.defined_vms.each do |name, subvm|
-        return machine(name, :virtualbox) if subvm.options[:primary]
+        return machine(name, provider) if subvm.options[:primary]
       end
 
       nil

lib/vagrant/plugin/v2/command.rb

@@ -110,7 +110,7 @@ module Vagrant
 
           # Make sure we're only working with one VM if single target
           if options[:single_target] && machines.length != 1
-            primary = @env.primary_machine
+            primary = @env.primary_machine(provider)
             raise Errors::MultiVMTargetRequired if !primary
             machines = [primary]
           end

test/unit/vagrant/environment_test.rb

@@ -111,7 +111,7 @@ describe Vagrant::Environment do
 
   describe "primary machine" do
     it "should be the only machine if not a multi-machine environment" do
-      instance.primary_machine.name.should == instance.machine_names.first
+      instance.primary_machine(:virtualbox).name.should == instance.machine_names.first
     end
 
     it "should be the machine marked as the primary" do
@@ -128,7 +128,7 @@ VF
       end
 
       env = environment.create_vagrant_env
-      env.primary_machine.name.should == :bar
+      env.primary_machine(:virtualbox).name.should == :bar
     end
   end