As advent, I love to use the Hanger Defense and research the tech that gives you TWICE the number of ships in a hanger defense. I will often build 2 or 3 of these on each strategic planet.
TEC and Vasari hangars hold 2 squadrons and Advent holds 3(not 4). But TEC and Vasari hangars take 4 slots, Advent takes 6.
also, make sure your research for turre*t defense fire power is at the highest level.
Vasari turrets shoot Phase Missiles. Increasing their firepower would also use the same technology that you need to increase the firepower of so many other ships/strikecraft. Unlike TEC Gauss guns which have separate upgrades, and have shorter base range(6700 vs. 7500 for Advent/Vasari).
In the end with all upgrades, each faction's turrets have a different advantage:
TEC - More raw DPS(thanks to faster cool-down of Quick Gauss Capacitors)
Vasari - Phase Missiles and punching through shields
Advent - Longer range from the Precision Beam Focus tech(7500 -> 8400 after 3 levels)
So. How exactly do you defend in this game (Barring using your own fleet) as Vasari?
You don't. Especially not as Vasari, who are offensively oriented. You must use your own fleet, or a smaller secondary defense fleet(with the side effect of reducing your "main" fleet's size) combined with orbital defenses.
Vasari are especially accustomed to defending with their fleet when they have to defend, since most of their tactical structures are actually pure fleet-support structures which do nothing on their own, like the Regeneration Bay, Nano Weapon Jammer, and Phase Stabilizer. Also, Hangar Defenses are more meant for adding a couple of extra squadrons to reinforce your defensive fleet, not fight the enemy on their own. The most they ever do on their own is take down a small/medium siege frigate rush or kill a passing scout.
Even the Vasari superweapon, the Kostura Cannon, is a support artillery gun for augmenting fleet attacks on enemy territory(and works best with carrier fleets). If there has to be any turtle strategy at all, than it's the Advent or TEC who'd be using it to build their anti-planet superweapons and then unleash a barrage of shots from behind their stronghold.