Hello,
Firstly, welcome to using Communities and VMware in general!
By "single Linux server" I am assuming you mean a single ESXi host running on a physical server with 6 VMs on it.
To access control of these host and the VMs (on the ESXi host via SSH) you will need to enable incoming SSH connections the steps for doing so is outlined here (for ESXi 5.1):
http://www.vmwarearena.com/enable-ssh-and-esxi-shell-for-esxi-51/
But to access this you will need the vSphere client for your local computer to access this GUI interface/client:
https://kb.vmware.com/kb/2089791
This *client* is more than just a GUI, it is feature-rich and you can do near anything between this and the CLI.
If you are unable to install this due to the security reasons specified (ask an administrator if you can install this, it will things a lot easier) then if you have physical access to the server you can enable SSH via the Direct Console User Interface (DCUI)(e.g. plug a monitor, mouse and keyboard into it), you can also access the CLI here.
Alternatively if the RHEL VMs are functional and the necessary ports open, you may be able to access and manage them directly via SSH.
Also, regarding 'vCenter' this is basically just another client/GUI for managing multiple hosts, if you have only one host (on one physical server) then this is likely not implemented in the environment. (this can be connected to using the vSphere client in the same way you connect directly to a single host using the application noted above)
Bob
-o- If you found this comment useful please click the 'Helpful' button and/or select as 'Answer' if you consider it so, please ask follow-up questions if you have any -o