Just saw this thread and it looks like you’ve already made a decision, so this reply is really for any others out there working on a similar project…there have been so many who have posted information that has helped me, so maybe this will help someone else.
Last year I moved into a house with an existing ADT system. The documentation calls it a 3000EN, but it appears to be based on a Vista20p alarm board. I already had an envisalink (EVL-3) from my previous house and it works with the Vista20p. However, I ran into obstacles attempting to integrate with openhab. The DSC alarm binding is only for DSC alarms and the alarm decoder binding requires specific equipment different than the EVL-3.
So, here’s how I got around this…
I found some python code written by MattTW called Honey Alarm Server (https://github.com/MattTW/HoneyAlarmServer). It is written in Python 2, but it integrates with Vista20p and EVL-3. It includes a simple webserver interface that can be used to arm/disarm and see zone status. The author included some example plugins for various home automation projects (openhab was not one of the them).
I’m not a programmer, but using the provided examples I managed to cobble together an openhab plugin that interacted with the honey alarm server code and used PUT commands to update openhab items through the Rest API. I installed the exec binding and can arm/disarm through openhab switches as well.
I run openhab on a raspberry pi zero, so I just installed the Honey Alarm Server software on the pi zero and configured so that it is initiated on system startup.
Admittedly, there is a learning curve to set this up, but it may offer an alternative to someone attempting to integrate openhab, EVL-3 and a Vista20p. It has been awhile since I set this up and I didn’t document all the steps, but if anyone is trying to do this, I’d be happy to try to answer questions to assist.