The sequence for adding a Master code or User code on a Vista 20P system are as follows below:
To change the Master Code: Master code +  +  + new code + new code
E.g  +  +  +  + 
To add a User Code: Master code +  + user number (03 up to 33) + new user code
E.g.  +  +  + 
To Delete a User Code: Master Code +  + user number (03 up to 33) + [#] 
To add an authority level to a User code follow the sequence below:
Master code +  + user number (03 up to 33) + [#]  + desired authority level
E.g.  +  +  + [#]  
The Authority Level will determine what the User is capable of doing with that code.
0 = Standard user (will allow the user to arm & disarm the panel at anytime)
1 = arm only (will allow the user to only arm the panel)
2 = guest/babysitter (will allow the user to arm the system but will render the user unable to disarm
system unless guest/babysitter code was used to arm the system)
3 = duress (this code will do a fake disarm of the system and silently notify the Central
Monitoring Station if you are under duress)
The sequences above have to be entered without any pauses during the keystrokes. Any pauses will result in the new code not being assigned.
Once the code sequence has been entered you will want to test the new code to make sure it works.
Turning your Chime On and Off is a good way to test a new Master Code. After you have set a new code rather than arm the system, only to discover the panel did not accept the code, simply turn the Chime On or Off with the new code. Its a quick and easy way to verify that the code was changed; without having an armed system and no codes.