Don't get me wrong, the idea of having Hyria directly under Eretria isn't a bad one and I would, would the circumstances be any different push for accepting it but as it stands right now they've rebelled not only against Artahias and the Messapit nobles because of the cattle drive but also against Eretrian rule. Accepting them as our direct vassal without some form of punishment against those who started the rebellion in the first place just isn't worth it long term.
Not when it would mean having the rest of Messapi as allies instead of vassals, vassals we can control and work to integrate in to Eretria as a whole in the long run, either through an open hand in the form of rewards and good will or the sword. We know Artahias is a cunning and ambitious individual, how long would it be until he thought he no longer needs his Eretrian allies and joins our enemies to oppose us? Messapi is too close to Eretria for them to be allowed to rule themselfs.
Between Hyria, Brention, and Caelia, that's the majority of the Messapii population; in fact, if my math is right, it's ~70% of the population.
Hyria on it's own is ~5,500 people and the Messapii are <13,750. All of the other cities are roughly of the same size and we know they have <5,000 people based on the map. If we assume (as is likely) they have a roughly equal share of the remaining population, each Messapii cities has a population of 1,000 - 1,500, on average. Assume Bretion and Caelia are average, then we can expect to have 8,250 Messapii vassals compared to Artahias' 5,500. Once you add in the 1,200 additional Kretian Freemen that moved into Brention with the Stayr, then it becomes 9,450 to 5,500; we have nearly twice as many Messapii freemen as Artahias does.
It's likely that there's also going to be a lot of internal migration as well. We know that Artahias is consolidating land and kicking peasants to the curb in order to build up a larger aristocracy and artistocratic cavalry. It's likely many of them will move into the lands directly vassalized by us as opposed to starving under Artahias' rule; land consolidation always involves displaced, starving peasants (look at the process of Enclosure in the UK). Hyria is effectively a revolutionary state that depends on small holding land owners, like earlier Eretria; it's likely they will find space for neighbours and kinsmen fleeing destitution in Artahias' domain. The population growth rate is likely higher among our subordinate Messapii and lower among Artahias', combined with the naturally larger population base and Hyria as a cultural center and Artahias will eventually be eclipsed by our vassals.
Splitting vassals into differentiated lumps is like chewing your food, it makes digestion easier. If Artahias is on the other hand undisputed ruler of the Messapii, it's much easier for him to try and rebel with the Messapii as a whole, either with Tarantine or Athenian support. 10,000 freemen are actually much more frightening under one man than that same number divided up. Leaving the Messapii in one solid, consolidated block means that there's less space for Eretrian culture to act on and influence them; they have a greater internal cultural cachet to resist conversion.
We have to break up large cultural blocks in order to start the process of integration; keeping everyone together in their own little ethnic tribe is not going to lead to a break down of national barriers with Epulia or Eretria. Rome was build great by expanding the definition of 'Roman'. If citizenship truly depended on ethnic status, Rome never would've expanded across Italy; you saw this with the Social Wars, Roman allies and vassals refused to work with Rome unless they could be Roman.
Edit: Considering the fact that Hyria is current using Hoplites and phalanxes for warfare, they are pursuing a very Greek path. They will need it and continue down that road as vassals to us as well. I sincerely believe the fastest way to Hellenize the Messapii is by granting Hyria vassalage and using their and Brention's status as quickly Hellenizing groups to exert cultural counter pressure on the rest of the Messapii, using their cultural center against them.
