vmsterm / vaxterm
A very nice terminal emulation using xterm and some tweaks.
It requires xterm installed on your computer 
From http://invisible-island.net/xterm/xterm.faq.html:
This script is for people who wish to connect from a Linux or Unix computer to a VMS computer using telnet and get a good VT100 or VT220 emulation. The key mappings have been specifically designed to emulate the VT terminal auxiliary numeric keypad, so that you can use VMS EDT and TPU editors, as well as the many VMS applications use keys PF1 to PF4. The script should work with any recent version of Xterm using a standard extended IBM PC keyboard or a Sun keyboard.
About the keymappings. First the auxiliary numeric keypad. My prime objective with these mappings was to produce a setup that I could use with the EDT and TPU editors which make extensive use of the numeric keypad. The top row of keys PC numeric keypad (Num Lock, Divide, Multiply, Subtract) are where you find PF1, PF2, PF3, PF4 on a VT keyboard, so I have mapped them to PF1 thru PF4. The PC numeric keypad Add key (+) takes up the space of two keys which are Minus and Comma on the VT keyboard – I have mapped it to Comma (Delete Character in the EDT editor). I have then used the PC Pause key to map to VT key Minus (Delete Word in the EDT editor). The remaining keys on the auxiliary numeric keypad are the same for PC and VT.
The six keys between the main and numeric keypads on the PC (Insert, Home, Page Up, Delete End, Page Down) are usually mapped to the VT keys by either position or by (approximate) function. As I rarely use these keys I have mapped them by function as follows: PC key Insert to VT Insert Here, PC Home to VT Find, PC Page Up to VT Prev, PC Delete to VT Remove, PC End to VT Select, PC Page Down to VT Next.
Function keys
There are 12 function keys on the PC keyboard and 20 on the VT keyboard, so I map PC F1 thru F12 to VT F1 thru F12 (except for F1 thru F7 as noted below) and PC Shift F1 thru Shift F10 to VT F11 thru F20. The VT keys F1 thru F5 are local hardware function keys so there is nothing to emulate, however some PC to VT emulations in the past have mapped PF1 thru PF4 here, so I have done that too, even though they are already mapped on the auxiliary numeric keypad.
F6 is now mapped to Keypad ENTER, that's very useful! F7 is now mapped to the 7 in the numeric keypad, useful to enter command mode in RT-11 EDIT pressing PF1-keypad 7!
Xterm functionality
You lose some xterm functions when you remap the keyboard, however this script implements a scroll back buffer of 1000 lines which you scroll through using Shift and Up (a.k.a. Up Arrow or Cursor Up key) or Shift and Down.
A summary of the keyboard mapping
PC Key maps to VT Key. ------ ------ F1 PF1 F2 PF2 F3 PF3 F4 PF4 F5 unused F6 Keypad ENTER F7 Keypad 7 F8 F8 F9 F9 F10 F10 F11 F11 F12 F12 Shift F1 F11 Shift F2 F12 Shift F3 F13 Shift F4 F14 Shift F5 F15 (Help) Shift F6 F16 (Do) Shift F7 F17 Shift F8 F18 Shift F9 F19 Shift F10 F20 Shift F11 F11 Shift F12 F12 Print Help (F15) Cancel Do (F16) Pause Keypad Minus Insert Insert Here Delete Remove Home Find End Select Prior Prev Next Next BackSpace BackSpace (sends DEL - ascii 127) Num_Lock PF1 KP_Divide PF2 KP_Multiply PF3 KP_Subtract PF4 KP_Add Keypad Comma KP_Enter Enter KP_Decimal Period KP_0 Keypad 0 KP_1 Keypad 1 KP_2 Keypad 2 KP_3 Keypad 3 KP_4 Keypad 4 KP_5 Keypad 5 KP_6 Keypad 6 KP_7 Keypad 7 KP_8 Keypad 8 KP_9 Keypad 9 Up Up Shift Up Scroll Back Down Down Shift Down Scroll Forward Right Right Left Left
And here's the script! 
How to use:
Usage : vmsterm -options
Options: -80 for 80 column terminal
-132 for 132 column terminal
-bg colorname
-fg colorname
-fn fontname
-fb bold fontname
-host hostname portnum
Example: "vmsterm -80 -fg white -bg black -fn 9x15 -fb 9x15b -host earth"
Starts a session with an 80 column terminal
with a black background, white foreground, a normal
font of 9x15 and a bold font of 9x15b, and connects
to the node 'earth'
Example: "vmsterm -host earth portnum"
Starts a session with default terminal settings
Example: "vmsterm -help"
Displays vmsterm options
How to use EDIT/KED editor under RT-11 with vmsterm
We can edit files in RT-11 using EDIT/KED, a visual editor for VT100 terminals. This involve use of numeric keypad and PF1...PF4 keys on the VT100 terminal. Main problem is: how to save/exit from the editor under a telnet session via terminal emulator?
We can create and edit a file as follow:
edit/cre file.txt
To save/exit:
press F1 (PF1) and F7 (Keypad 7) to enter command mode
write exit and press F6 as ENTER (Keypad ENTER) instead of normal ENTER key
Useful commands are write, exit, there are other commands described in the manual linked under References section. Every command must be terminated with the numeric keypad ENTER, so pressing F6.
References
* http://invisible-island.net/xterm/xterm.faq.html
* ...