As I commented in the other thread on this topic, this kilt outfit is for a child, and it was not uncommon in the recent past for boy's kilts to be made with an attached vest. I suppose it made fitting a growing child a bit easier. I don't see this done too much today, but it was once pretty common practice.

(I wouldn't use it as a precident for a grown man wearing his vest tucked into his kilt, though! That would look a bit odd indeed!).

Matt