On every VerneMQ node you'll find the vmq-admin command line tool in the release's bin directory (in case you use the binary VerneMQ packages, vmq-admin should already be callable in your path, without changing directories). It has different sub-commands that let you check for status, start and stop listeners, re-configure values and a couple of other administrative tasks.
vmq-admin has different sub-commands with a lot of respective options. You can familiarize yourself by using the --help option on the different levels of vmq-admin. You might see additional sub-commands in case integrated plugins are running (vmq-admin bridge is an example).
vmq-admin works by RPC'ing into the local VerneMQ node by default. For most commands you can add a --node option and set values on other cluster nodes, even if the local VerneMQ node is down.
To check for the global cluster state in case the local VerneMQ node is down, you'll have to go to another node though.
vmq-admin uses RPC to connect to some node. By default, it has a timeout of 60secs before vmq-admin terminates with a RPC timeout. Sometimes a call (for example cluster leave) might need more time. In that case, you can set a different timeout with vmq-admin -rpctimeout timeoutsecs or even -rpctimeout infinity.
vmq-admin is a live re-configuration utility. Please note that all dynamically configured values will be reset by vernemq.conf upon broker restart.
As a consequence, it's good practice to keep track of the applied changes when re-configuring a broker with vmq-admin. If needed, you can then persist changes by adding them to the vernemq.conf file.