Vertex snapping is barely usable in Unreal, it’s correct that you have to press V but it only works on some meshes:
As you can see it’s trying to snap the object to the landscape or to the cone (the cubes and the brushes are completely ignored), you don’t have the feature turned off, it just sucks 
You can also try to assign a shortcut to “enable vertex snapping”, it seems to work better, but it’s not a great improvement:
