Is it a 4x4.....?
My 2007 Vigo auto was a 4x4, the only Vigo that could be bought with an auto at the time.
In gear and stationary, the internal parts of the converter are not doing much at that low idle rpm, the converter casing is continuously driving the box oil pump, which it will do in or out of gear. I have only ever put an auto into "N" if I want to take my foot off the brake and use the handbrake, perhaps a long traffic stop.
My personal thought is that continuously knocking in and out of gear puts unnecessary stress on the drive train, perhaps over a period of time it will show in the mechanicals.
Your ride has a split drive shaft, it has a bearing in a rubber lined mounting, check it out, check the fixings, it maybe worth checking that the torque converter/flexplate (flywheel) fasteners are tight.
The other thought is the lock-up part of the converter, but I have had no dealings with those to comment.
I can't see how your garage can give you a job price unless he knows what the problem is..Unless he gets a box from a right-off, or he has a recondition-er contact...