As I recall, part of the dynamics that led to the vassalization of the Peuketii was that after fighting us repeatedly, losing their coastal cities and being driven inland, and then losing
again a few more times, the Peuketii were so reduced in strength that they became more vulnerable to the Lucani, who started raiding them more aggressively.
At which point they were caught between hammer and anvil, and a subservient alliance with Eretria to avoid being pillaged by the tribes of the interior started looking better, I suspect.*
If we take over 'just enough' of the Dauni territory to secure our access to Lake Salpi for the salt trade, I suspect something similar is going to happen. Based on events before the war with Taras, we might hope/expect to integrate Salapia and Herdonia relatively peacefully as vassals, but there would surely be a war against the Dauni further inland- at least one, probably more than one unless the war is
very decisive.
Assuming this goes well for us, the Dauni will be weakened enough to become a more inviting target for future raiding by their neighbors, who may start growing at their expense until the Dauni are either outright conquered and destroyed as a people, or forced to ally with us in self-defense. Either way, it ends with us bordering the Samnites, not with the Dauni acting as an unwilling buffer state- because the reduced strength of their kingdom/confederation won't be able to survive in that role while also trying to get revenge against us and against the 'traitor' Dauni towns.
Another issue I foresee is that large-scale salt extraction is
proverbially unpleasant work, usually not performed by freemen- "just bring in more metics" isn't necessarily going to cut it, and there may be demand for more slaves or serfs among whoever is running those salterns. That could drive some ugly activity.
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*[
@Cetashwayo , was there any fighting between the Peuketii and various inland Italian natives during the timeskip? I'm curious, if nothing else.]