Back on the case
Damn, that's a fun setup

Lot's of possibilities, and ground to cover.
OK, to avoid gibbering, I'll try to divide the two main themes: power-vacuum and the drug eco.
Right now, it's a scarcely inhabited, badly protected, low-profit area nobody gives a s. about. A nuisance heirloom; a place that wine comes from, the kind that marshals and squires drink. With stories of brigands ruling over countryside, it's a boogeyman to threaten officials with. Be good, or you'll get "promoted" to a barony in that s

hole.
But what happens when the bandits are gone, there's a somewhat centralized law, and there appears to be a steady cash influx? Suddenly it's a charming, sunny area with booming economy, and the kid wants it's toy back!
At first, there there would probably appear some agents of king, just scouting out the situation, probably posing as merchants with escort. Then, when it's time to reclaim the area - sheriffs to "relieve" self-appointed law from it's duties, thanking them for good work, go back to whatever you do, peasant.
The people could just cave in, there could be frictions or even a rebellion. They could find a "front noble" to pose as if they had a rightful liege in place; said noble could really be a puppet or could use the situation to really grasp the reins of power. With time they could try and go independent.
The rebuild, and drugs angle:
Ideas for startup quests:
- is everyone and everything accounted for? Check who is down or dead, what about the main structures. City hall could be rigged with traps, with a couple of bandits holed up, desperate to think of a plan as to what to do now. A person or two counted among dead could actually still rescuable.
This could serve as a good introduction to the area post-attack for the party.
- The fruits are ripe, and will fade in a couple of days. All hands are needed at gathering, but there's so many injured! The party needs to find some medical help for the townsfolk with their limited resources. There are several possibilities in the area, but the player's have to decide whether to get a great and knowledgeable cleric, which will allow to help lives of dying man who used to be herbalist and pharmacist, as well as X (insert an NPC here

); or more help, that is less knowledgeable - but will be able to more or less mend all injuries, thus minimizing the loss of yearly crop.
The choice here (and whether the chosen path is successfully executed) will also affect the struggle between vineyard and drug people. If majority of crop would go bad, there's a lot more incentive to go into drugs, even if "only for a little while". Death of pharmacist on the other hand, while bad to the town, would slow progress the drug people can make, or just throw them a tier lower cash-wise, as they could provide only raw material.
The road to drug producers can be slow and blurry. Depending on how you like to run campaigns, it can be corrupt baddies vs hard-working goodies, or a situation where everyone has their own motif, and believes to be in the right.
The herb they're gathering could be quite mundane, but not often used, something like salvia (and the drug that's made out of it). From their perspective, it's a herb; a weed almost. They gather it, and process some of the way. If some rich folk wants to pay a lot of cash for it - who are they, to forbid them to?
They know how alcohol can be abused, while they never saw any salvia-induced bar brawl. ;-) In time, this would change, some of the processed drug would inevitably find it's way back. But by that time it could be far down the road.
And just go baby-steps. All the work for now is done, the stuff is gathered and processed, waiting in bags in several points in town. On the other hand, the vineyards were unattended to, the city was badly damaged, and there is winter to look for. I mean, they need the money, so why not just sell what they got, and use the money for rebuilt? Note, that based on PC choices and allegiances, they could be the ones making contact with buyers, or it could be some faction from town.
Now that the contact is made, and they know it's extremely profitable and easy to cultivate (as it's basically a weed). It's not really a choice of whether to do it or not - but rather if it will be out in the open. If the city decides to ban the business, inevitably some crimelords will make contact with the disgruntled minority, to get them to provide it again. If that happens, it will be in smaller amount, but power of the outside crime will slowly creep in.
If it can be done out in the open on the other hand, more money and power will remain with the town, but the transition from hard work in vineyards to harvesting the drug will be swifter.
Some possible quests here: a drug harvest delivery. The important thing about drug exchanges (especially if no side is dependant on having one contact to deal with) is that they're
always a stakeout. The trick is, that if both sides know this, and are well prepared for it - it becomes a silent, latent mexican standoff. If one side is unprepared, you take all their stuff. If you're the unprepared one, you try to back out as quickly as possible, maybe not deal at all. If there's an equilibrium, an exchange takes place, and all part. This can be hard both on players and you, and be quite impossible if players are too trigger happy. Even if they're not, for some bizarre reason there's like a 80% chance that they'll get - even if just this once, and screw it up. So don't rely on only one organization and that it'll go right the first time around. You can of course influence their chances by adjusting how desperate given organization is for the delivery, and thus - how much firepower they'll allow PC's to have, and how many opening they'll pass over.
Another hook would be to have PC's investigate what's the big deal with the drug actually is. Bizarre interest could be truly abstract to the citizens, and they'd have to go to some big city and see what the drugs are, giving them a possibility to witness how people escape into addiction. Depending on how throughout they are, they might see a bunch of yuppies apparently having great time, a noble/wizard/bard taking a sniff every now and then to "clear his mind" or get to people ruining their lives, stealing from family for another fix, and laying in gutters.
BTW - I didn't process the idea entirely, but this can be adjusted to history and situation of Kowloon Walled City. Long story short - it was a working anarchistic settlement. After a time they were taken over by the Triads, at which point police from outside made a series of raids, and got rid of them; from this point there was no organized crime (though plenty of petty criminals ;-) ). It was an amazing place, and at very least it's interesting to read about. But I'm fairly sure it could be adapted to this situation (just toss off the architecture

).
Cheers!