The type of nic can be set for bridged network.

:nic_type can be specified in the network bridged type. Its value can
be Am79C970A, Am79C973, 82540EM, 82543GC, 82545EM, virtio.

lib/vagrant/action/vm/network.rb

@@ -354,7 +354,8 @@ module Vagrant
             :adapter     => config[:adapter],
             :type        => :bridged,
             :bridge      => chosen_bridge,
-            :mac_address => config[:mac]
+            :mac_address => config[:mac],
+            :nic_type    => config[:nic_type],
           }
         end
 

lib/vagrant/driver/virtualbox_4_0.rb

@@ -112,6 +112,11 @@ module Vagrant
             args.concat(["--macaddress#{adapter[:adapter]}",
                          adapter[:mac_address]])
           end
+
+          if adapter[:nic_type]
+            args.concat(["--nictype#{adapter[:adapter]}", adapter[:nic_type].to_s])
+          end
+
         end
 
         execute("modifyvm", @uuid, *args)

lib/vagrant/driver/virtualbox_4_1.rb

@@ -112,6 +112,10 @@ module Vagrant
             args.concat(["--macaddress#{adapter[:adapter]}",
                          adapter[:mac_address]])
           end
+
+          if adapter[:nic_type]
+            args.concat(["--nictype#{adapter[:adapter]}", adapter[:nic_type].to_s])
+          end
         end
 
         execute("modifyvm", @uuid, *args)