The Imperial Edition download from the Vault was just a temporary measure while GOG and Larian fixed the problem with people getting the regular version. From the
GOG DC release topic:
Anyone who backed the Original Sin Kickstarter and claimed his or her copy of Divinity: Dragon Commander on GOG and *didn't* get the Imperial Edition code issued, please contact [GOG] Support and we will give you a free code for the DLC. In the subject of your ticket, please put, "Imperial Edition Upgrade for Kickstarter Backer".
Do you have the 'Planet Earth' map at the bottom of the map list for a multiplayer or custom single player campaign? If so the Imperial Edition is installed. With a patched game there is checkbox in the gameplay options to 'Enable Golden Dragon Skin', which you can check/toggle to see if that will get it to show up during RTS. There is also a golden jetpack option, if you got the retail version of the
Divinity Anthology or signed up for a promo Larian had a couple years ago.
The rest of the Imperial Edition content (soundtrack, etc) is available from the install folder.