Thank you for your nice comment and for considering the proposition!
When you hold V the vertex of the selected mesh close to your mouse shows a box, if you release V that box vanishes. Instead of clogging the whole interface with a lot of boxes (one per vertex) they only show one, pretty neat actually.
When you found the one you want you can then drag it and while you move it that vertex will snap to the closest vertex under the mouse, so you have a real time preview of the position where you drop it.
Conserning the Z axis it will still snap on vertices hidden behind the pointed mesh. In Unity it sometimes snaps very far but the real time preview avoid this being an issue, just move the mouse a bit untill you have what you want.
Here’s a gif that shows exactly how it behaves, its very clean and convenient as you can see.
This should be by default in UE imho, I wouldn’t be surprised to see this feature implemented in the real build down the road because unity and maya users are going to ask for it a lot and unless snaping meshes at floating point coordinates causes some performance drop there’s no reason to leave this as a plugin forever no?