It’s actually a curious fact that neither Sidious nor Tarkin ever gave Darth Vader access to the Death Star’s blueprints… just think about it: this is the same man who, by the age of NINE, salvaged and rebuilt a droid, almost restored a destroyed pod by himself and regularly tinkers with his personal starfighter and his own suit. Sidious, at the very least, should be aware of Vader’s capabilities as a mechanic and what he could do to improve or fix the battle station. The only reason I can think for Sidious to deny Vader such access is fear that he would eventually use that knowledge to seize the Death Star for himself or build his own weapon to overthrow his master, which didn’t seem to be an issue at the time, as Vader didn’t consider overthrowing his master until he found out about his son, Luke.
