They can probably get the data easily. Having the data is very different from having the industry to take advantage of that data. Moreover, what evidence do we have that the Hegemony even wants to build Vulcans? We might think it's a good idea, but that in no way guarantees that the Hegemony leadership feels the same way.
Weirdo between having heavy fighters and not having them and eternally depending on the importation of this or that supplier who in this or that year puts an embargo on you because you did something that he did not like, there is not much option or much to think about, particularly when your adversaries have DS, PWS and Fighters all over the place is common sense.
They just haven't decided anything about the production of the HMI since Field Report 3085, or from Marian Arms Inc in 3145 or from Techwizards who may well have retroengineered since they are specialists in that and in rebuilding something piecemeal, the options that there could be any mech or Aero in the Hegemony you have it there, and not only for the MHAF or the Militias but for the Fields of Mars of each system or even for Patricians who want to have their mech in perfect condition or directly a small group of paid Privatters to take care of their possessions, let's say that if this is common in the rest of the IS and the Periphery I do not see why in the Hegemony it was not the same way.
Because this happens, it is simple little or no internal development of the faction, post the MPS book.
As the update is relegated to Era Reports and Field Manuals, the existing Fluff is scarce or null, since it is not that we have appeared mentioned too much or well in the novels that have come out.
Tangential mention in Star Lord, in the novel of the Avanti Angels Double Blind as the puppet antagonist of WoB, and some mentions to the step more.
If at least we were good protagonists in some novel, perhaps we would have more material to see well what Marian society is like, since there is none of that, at my tables I grab what there is and mix it as Republican Rome / Empire was and that's how they act