I dont think you're reading Vladimir lore correctly. He never "left" his Darkin. What actually happened was the following:
- Darkin conquers Vladimir's father kingdom and takes Vladimir as hostage to ensure his father's loyalty.
- His father betrays the Darkin anyway as he didnt care about Vladimir.
- The Darkin orders Vladimir to lead his armies, scaking the city and retaking it on the Darkin's name
- Vladimir govern his father's kingdom on his place, possibly suffering abuse from the Darkin and knowing his inhuman strength, his loyalty is seemlighly assured. On Vladimir own words:
"I was a good and faithful hound on a leash.”
- Amused by Vladimir's devotion and loyalty, the Darkin teach Vlad blood magic more for fun than anything. Vladimir grew to govern almost as merciless and cruel as the Darkin himself. But considering how the Darkin seemed Vladimir as just another mortal that he could smash whenever he wanted, most of that was made more for fun than anything.
- After Twilight of the Gods, countless of once slaves unite and march upon the remaining Darkin, with the help of the Aspect of War and the power of Darkin weapons on the hands of mortal heroes. On desesperation the Darkin order Vladimir to lead his armies on his defense, but then Vladimir betrays him, the Darkin's warhost together with some of the other rebel armies fight the Darkin. And then Vladimir lands the killing blow and absorbs part of the Darkin's power though blood magic.
Then later he allies with LB, helps to overthrow Mordekaiser and found Noxus etc etc.
But to clarify this, Vladimir never escaped the Darkin, he killed his master and absorbed his power(which is the source of his great magical power and domain over blood magic, he was extremly weaker than his current form during his mortal life). The next Darkin weapon(which isnt necessarily the last one) not being mentioned yet doesnt really needs an in lore explanation beyond "We didnt reach the point where it's revelant yet" we will eventually get info about it, we just need time. Also Noxus was founded 550 years after the Darkin were defeated.
About why the Darkin would let him govern and lead armies and all, it wasnt because of special interest in Vladimir, the Darkin just thought he was above meral mortal tasks. The Darkin didnt see themselves as kings, they saw themselves as Gods, they didnt conquer nations to govern them, they conquered nations so every mortal there could bow their knees, praise and fear them, simplying serving as tools on their hands. Tasks such as governing and leading armies arent things Gods have to do directly, those are just tasks their subordinates have to do on their name. And also, the Darkins couldnt be everywhere at once, when they were out conquering more nations and fighting other Darkin someone had to be on the nations they had already conquered to make sure those nations wouldnt rebel and would keep suplying the Darkin armies with food and soldiers.
