First you will need to click on the the Start Menu (Vista Globe). Then select Control Panel. From here you will need to select Programs and Features in the Control Panel window.
The Programs and Features window
Now in the side panel in the window you will need to select Turn Windows Features on or off. You will then seen a window pop up with a list of a bunch of things. You will need to scroll down to Telnet Client and then put a check in the box and then click OK. From here Windows Vista will install Telnet on your system.
Windows Features window
Now you should have Telnet installed in Windows Vista. The installation may seem like it is stalled but it will install after a bit or at least it did on my machine for a little bit. Have fun with Telnet.