For several hours I tried to add another field and modify the structure of the Roots vCard. I succeeded partly, but I failed adding another field above the street address.
What more is there to modify than what’s in widgets.php?
Sorry, all good. I resolved it. The problem was sitting in front of the computer… 
And the solution was?
The solution was that I was tired and therefore didn’t see the obvious. Adding/Removing fields is done at the very top of where the vCard Example Widget starts (line 30), arranging the data you do from line 86 on, referring to an untouched widgets.php.