Okay, you have two options:
(1) Find a User Code by brute force: Take up a new hobby, of trying x-hundred 4-digit codes a day, starting from 0000 and working toward 9999. You can try some obvious codes first, like 1234 (you would not believe how many homes I've found that still use that factory default), then the street address number if it's 4 digits, then google "most common PIN numbers". You might get lucky.
This is actually the simplest fix. It will take time and patience, but it saves you from going through the learning curve of having to program a new control panel from scratch.
(2) Which brings us to your 2nd option, which is to buy a new Vista panel off ebay for about $20-25, and programming it from scratch. In order to program it, you would need to know what devices are in the system and how they're configured at present.
It's in the nature of alarm systems that every system is customized. There is no "one size fits all" in professional-grade systems. Hopefully you have a zone list written down somewhere, usually on a card in the door of your keypad(s), or written inside the control panel cabinet door. You'd need to know what peripherals you have. RF Receiver for wireless zones (numbered above Zone 9)? With only 6 hardwired zones, it's likely that doors and windows are grouped on some of the zones. You'd probably program your usual entry/exit doors differently than back door(s) and windows, _if_ it's not on the same wired protection loop.
I'm not going to try to make a comprehensive list here, just wanted to warn you that changing out the panel is not hard, but programming can be a PITA for a novice. That's why I say that brute-forcing a User Code, although it may seem tedious and boring and take time, may actually be less trouble.
If you go the code-search route, here are a couple of pointers:
(1) Disconnect the siren/sounder wire(s) from Terminal 3 (T3) on the panel. Spare the neighbors. Unfortunately, the keypad(s) will sound during an alarm, but they'll time out in a few minutes (depending on programming) if people aren't coming and going and re-tripping the alarm.
(2) DON'T speed-dial the codes. The data processing in these panels isn't fast and it's actually possible for a normal person to machine-gun a code in faster than the panel can process it. You could enter the right code too fast and the panel won't recognize it. You don't need to be glacial, but dial deliberately. I'd guesstimate a little over a second for one code attempt (a + b + c + d + 1), or about 50/minute once you get a rhythm going. Listen for that "beep!" that tells you the code worked.
(3) Keep a written record of how far along you are. Maybe make a note every hour. It's ridiculously
easy to get distracted and then realize you don't remember how far along you were or the last code you tried.
(4) If/When you hit the right combination, STOP!! DO NOT test by arming again and seeing if it will disarm. Do not arm again at all! Test the new-found code by entering C-O-D-E + 9 and seeing the CHIME function toggle on and off. It's displayed on the keypad. Or enter CODE + 1 and listen for the "beep". Then let me know you got it disarmed, and I'll talk you through how to ensure you don't accidentally get yourself accidentally locked out again. Which can can and does happen. and may even be why you bought an armed system in the first place.