The home we just bought has an Ademco Vista Series alarm system throughout the house. The seller left us a remote to turn off a ringing alarm can't provide the "master" code that the User Guide states is necessary to reprogram with a new code. What to do? Whom to call?

Any help greatly appreciated.

You can call the company he used or almost any alarm company you want. Who are you going to use for the monitoring company.

I will give you may one opinion don't use APEX,

Their shoud be a Law, I see many people that own their system, Monitoring Contract is completed, and they basically force the new owner to go Continue with their company, or you don't get the code, They should be required to change it to Default, or a code the owner can use. They do it down here as well.
It's like Extortion.
If you are going to have it Monitored, they should be able to reset.
If you are not going to have it Monitored, Try a Local Small Company. Ask for a price up front. A simple system can be programmed quickly(No Wireless, No expanders, no addressable, etc.).
More complex systems take a little to figure as some things may not be located(remote expander?)
See if you can get a Model Number.

I own a PERS system. Still in monitoring. Complicated: Bought from subsideary of mfr, preprogrammed. They outsource monitoring.

It can be programmed over the phone. I asked for the password. They gave it to me. They just don't want it to stay the default value.

What I found is The Big Companies won't give their Company's Installer Code to you, then you would have the Code for all their systems.
You don't want to leave the default code.