You’re buying a place with home security devices
A new home may come with a security system. All the alarms and cameras work, are installed in the right places, and are ready to go. In this case, just turn on the phone to call and start a new service contract. A Vivint professional will still come out to ensure there are no usability problems, and they may offer ideas on where you should replace bad outdated components. It's a quick visit, and you can get your alarm monitoring set up without worrying about any new devices -- unless you get some new devices.
The outgoing homeowners can move an existing Vivint contract to your family for a nominal set-up cost to make your life easier. If the outgoing residents are downsizing to an apartment or smaller home, transferring the Vivint contract might be a part of the sale.
You are looking to move and take your home security equipment with you
You might want to bring your existing smart devices with you to a new place when you move. It should be pretty easy to transfer, as most alarms and smart devices are portable. You can take down each component yourself or use a professional installer to come and take care of the system. Then a pro will hang the same components -- and any smart items you want to add -- in the new place. Your home security contract will stay the same.