There are two things going on, in my assessment:

The first is that vandas are really vines, so the loss of lower leaves is perfectly natural as the plant grows.

I believe the second is the need for more water. Vandas are really thirsty. I like the concept of repetitive dousing, but two, two-minute sessions isn't cutting it, even if repeated in the afternoon. I suggest dropping the afternoon session and increasing that in the morning.

When I was still growing vandaceous plants in a greenhouse in PA, overhead watering was for 30 minutes at 5:30 in the morning.

More speculation: bigger plants have bigger reserves, so any stress will be more easily handled (without as much lower leaf loss).