If you want your Vagrant box to pull from the same DHCP your host box does, use this line in your Vagrantfile:
config.vm.network :public_network, :auto_config => true
This corresponds to a VBox bridged network, which it sounds like you want.
edit: added the auto_config bit. It should ask you which adapter you want to use when it boots; you can also specify :bridge => "en1"
(or whatever your adapter happens to be named; en1 is my Macbook's USB ethernet) on that line to hard-code a host adapter name.