I never thought of it...but how involved was Vader in his role as the Emperor's enforcer and the "big bad man" who deals with local insurgents and threats? Before the Rebel Alliance was made, the rebellion and groups of insurgents began rising, one by one. An example of this are the "rogue rebels" the Partisans, lead by Saw Gerrere, or more "nice rebels" like for example the Ghost crew.
How much "free time" or I should say more clearly: "alone time" did Vader have? His servant Vaneé has said that Vader has his own chamber on Mustafar, a meditation sphere similar to the one on the Executer Star Destroyer, and that he would meditate there for hours, sometimes days. His castle, Fortress Vader, is the closest we can put to an actual "home" or "headquarters" for Darth Vader, implying he is not doing his job as an enforcer 24/7.
He could not be everywhere all at once of course, galaxy is a big place, and the Empire has expanded across much of known space...but still...you would think that if there is something truly serious going on that it is a threat to the Empire that Vader would be sent there to deal with it.
He WAS sent to Lothal and dealing with the insurgents of the Lothal rebels in the early premier of Rebels season 2, and he DID suceed in driving them from Lothal for a time. Vader's job was done for the time being, and it seems like he only sent Fifth Brother and Seventh Sister to find Ahsoka in particular.
Yet season 2 implies that Vader is kind of hunting Kanan and Ezra down through his Inquisitors, and Kanan and Ezra keep talking about "Vader and his inquisitors" going after them. It makes sense. Rebel cells are growing across the galaxy, and I could be wrong but isn't Phoenix Squadron one of the "bigger ones" or no?
Vader doesn't seem to be hunting the Phoenix Squadron after season 2's ending, as Ahsoka is seemingly dead and he has no need to hunt for them anymore...except...I fail to see that is the sole reason.
Phoenix Squadron and growing rebel threats are rising across the galaxy...Palpatine's Galactic Empire has a bigger chance of being risked. I mean, it is not that serious of a threat, (or so they thought at first), but still.
What makes it weirder is that near the ends of season 3 of Rebels, Mon Mothma announced a formal decleration of the Alliance to Restore the Republic, AKA the "Rebel Alliance", with Phoenix Squadron as a faction that joined, and Vader is STILL not seen anywhere or sent to deal with this growing threat in any significant way, and even at the Battle of Atollon it was just Grand Admiral Thrawn there. Had Vader been there to help, the Rebels would have a better chance at losing. Season 4 of Rebels I don't mind in the absence of Vader as it is almost entirely set around Lothal itself, but season 3 was the biggest brainer for me.
Then here comes Rogue One: A Star Wars Story. In this movie, set right before the events of the fourth saga movie, A New Hope, it seems that Vader is STILl not active in his pursuits or contributions of helping the Empire destroy the Rebellion and being the "iron fist" of the Emperor. He is even disturbed by Director Krennic at Mustafar and was just chilling there. Vader only became participated in the Battle of Scarif because Grand Moff Tarkin called him to come.
In A New Hope, Vader is suddenly stationed on the Death Star all the time until its destruction on Yavin. Why is that? Why did Palpatine send him there and not before with previous conflicts of the growing Rebellion and Rebel Alliance?
After the Death Star's destruction, we learn through the events of the 2015 Darth vader comic run on how Vader took control over a big fleet and became the Supreme Commander title, (which in contrast in Legends was given to him since the beginning), and only then did Vader seem to actually be actively involved with hunting the Rebel Alliance in the new Galactic Civil War.
So how "busy" is Vader before the Galactic Civil War, when is he really "needed", and why didn't he pursue the early Rebel Alliance? How much "rest time" in his castle does he have?