A contractor installed the subject Viking refrigerator when my home
was remodelled last year. The remodeller is nowhere to be found. The
unit is not covered by the warranty either because it was a display
unit.

The refrigerator door does not close air-tight because the the door
and the refrigerator body does not align properly. Do you know any
practical solution to the problem? Could a hinge be the problem? If
so, what can be done for that? For a practical solution, I will offer
$30 fee. Thanks.

Could it be that the refrigerator is not resting level, one corner
sagging lower so the the front of the box is not plane, that is,
skewed, so that the door does not fit closely?
Such kitchen equipment have  (usually have) one or more adjustable
"feet", that you can screw to make them stand level.  If that is the
problem, and the fridge does not have such, you can shove a wedge or
or whatever under the "odd" corner.
If that is not the problem/solution, well, ...
Good luck

Hi,
I should have thought of this before.  If the fridge's door has a
front to fit in with the style of adjacent cupboard doors, the problem
could be the hinges, especially if the fridge door hangs on the front
that has hinges.
These can usually be adjusted.  
If this is the case, maybe my other suggestion could also apply, but
an experience handyman (you?) or a carpenter should be able to help.
Myoarin