What's important to remember is that, by the game's own lore, the Vlandians spent many years serving as auxiliaries to the Empire. This explains why they have a troop tree that is very similar to that of the Empire line, but also why they have attained certain technological innovations as it concerns warfare. In the game, both the Vlandians and Imperials have capes, crossbows and segmented armor whereas all the other faction primarily wear lamellar or chain mail (no one else has crossbows or capes).
However, this cultural exchange seems to have stopped at the battlefield. The Vlandian Towns are far less established than Imperial towns, save for Galend which I can only imagine was once the imperial capital of now Vlandian lands (formerly Baravenos) before its capture by the first Vlandian King, Osrac Iron-arm.
So Imperials still retain their affinity for engineering and maintaining large urban centers, whereas Vlandians have yet to establish a system for producing such learned individuals. I must underline that this does not make the Vlandians dumb or the Imperials smart, but rather it reflects priorities. The amount of work and training needed to produce and maintain individuals as learned and capable as engineers and architects in ancient time was actually equitable to that needed to produce and maintain knights or skilled household warriors. The Calradian Empire, as was the case with the Romans of our world, simply had a longstanding foundation and the resources needed for producing learned individuals even as they maintained their defenses. In a period of tumult, as there is greater need for warriors, there will be less emphasis on other fields. So in the present and until they become modern Swadia, the Vlandians will focus on perfecting their capacity for war - so that by Mount and Blade Warband we have the famed Swadian Knights and Swadian Towns that don't come close to matching the glory of the now ruined Imperial Cities of present Bannerlord.