Why is only half of the object covered when I apply decal?
I've tried with different work planes too but in vain. I'm uploading the .ipt file. If there is any designing defect please let me know and its remedy too.
OK first of all you made your Vase far to complicated.
Secondly your image was on the centre plane to use it as a decal you have to put planes on the right and left side of your model then new sketch with image then decal both to nearest surface.
Decal rarely covers 100% of the model its better to use your image as a texture
1 Other answer
You need to wrap the decal, have a look at this screencast that I recorded: https://autode.sk/3ffwTn2