Vasari's isn't about pure weapons, but about their ability to manuever their forces and run away when a superior force shows up. One of their capitals (the egg) is even described to be a flying city, hardly a military ship
Every race has a civilian cap ship (Akkan, mother ship). Whats your point?
Lore wise, the Vasari might be running away, but they have been rebuilding their empire in Trader Space. They are still slavers and still an oppresive empire, hence the name of many of their upgrades. They emphasise on geurilla combat because they are outnumbered, but they are militarily superior to the TEC.
If they were superior, they wouldnt have had to fight an drawn out war like this. From a militaristic point of view, the TEC is in the better position. They do have an established infrastructure and organisation and their technology base, while crude, is holdung up to the more advanced technology bases the other factions employ.
The whole Laser/plasma/shield skipping missiles thing can only get you so far after all.
Yes, put 2 kodiak vs a crusader/enforcer and both the enforcer and crusader will win, but when it takes you 3 weeks to replace one of your shiny strong ships while the TEC replaces 2 lost kodiaks with 3 in 1 week, you will be hitting a brick wall at some point. In terms on Military might, the ability to recover from losses is as important as the ability to inflict losses upon your enemy. Just because you have a miniscule advantage in the tactical theater doesn't mean you're in a good position on a strategic scale. Essentialy that has happened now. The TEC stubbornly refused to bail over and die an now the advent and Vasari are facing the entrenched hoomans.
The advent needs time to reorganize and stabilize its gains, while the vasari are in a rather devlish dilemma:
Stay to fight and hope to get enough out of it to make up for the investment or continue with the whole running part, because the clock is ticking. (thats also why they do not build an empire. they EXPLOIT the region then leave it once its stripped of all valuables, or at least try to.)
The whole guerilia shit doesn't fly properly when YOU are the attacker and be facing a wall around every important thing.