Like Nagios (which is where it was originally derived from), Icinga2 has the concept of a central server, with agents on other hosts that it can keep an eye on.
We're going to install Icinga2 on our centos2 VM, and then monitor our debian2 VM from our first host.
To get started, jump on to centos2 and install Icinga2:
$ vagrant ssh centos2 -- -L 127.0.0.1:8181:127.0.0.1:80
Note the forwarding section; this will be for the GUI setup later on (port 8181):
$ sudo yum install epel-release -y$ sudo yum install centos-release-scl -y$ sudo yum install https://packages.icinga.com/epel/icinga-rpm-release-7-latest.noarch.rpm -y$ sudo yum install httpd icinga2 icinga2-ido-mysql nagios-plugins-all icinga2-selinux mariadb-server mariadb icingaweb2 ...