Unless specified otherwise in Vagrantfile, the IP address of the host (the computer running Vagrant) from the perspective of the guest
(the VM being run by Vagrant) is: 10.0.2.2
If that IP address doesn't work, then examination of Vagrantfile should reveal directives that changed it from its default value.
If an IP address can't be found in Vagrantfile, then the following command will probably reveal it:
route -A inet