Not familiar with the Brinks system. I am guessing the alarm panel is a particular make? Typically these panels need an automation module or some sort of add on to interface with Vera. Concord 4 panels required a Superbus 2000 RS 232, along with a specific type of cable.
As far as running the ecolink, sure you can run it alongside the old wired sensor. Not really blending systems as you know.
If I understand your next question right, you want an old wired window sensor to trigger the ecolink that is next to the wired door sensor.
I caught myself thinking you should go to lowes and see what IRIS zwave stuff is on sale and just get a zwave siren, and window/door sensors.
Linking with the brinks system is beyond my ability. I pulled off the concord 4 integration. Just not sure it is possible with the brinks panel.
I would not trust zwave/vera as a total security system over a traditional security panel. (had to say it). So, I would never tell you to throw out your brinks system unless you swapped it out with a newer panel....but then we are talking some dollars.
Good luck. Sorry I was not much help.