Where can I use the shovel?

[Engineer In A Box]

I have made a shovel after hearing that an island I saw had a buried vault in it, but I haven't found any new islands which have soft enough soil to dig, and I would like to know what soft soil looks like if it looks any different from the regular types of soil.

[xXICRANKXx]

The bottom looks like piles of sand. Only the heaps of earth are more likely to be found in caves on somewhat larger islands.

[Xas]

In the beginning, dirt is all your shovel can be of used.
https://steamcommunity.com/sharedfiles/filedetails/?id=1647979181

As for your worded ' vault ', you need to progress further into the story,
upon Caravan Island, then you could use shovel together with another tool to dig for treasure.