Jikara Navigator
The basic scout ship for the Vasari, the Jikara is the only ship they have that isn't notably more powerful than similar vessels belonging to the other races. Ultimately this doesn't matter - it is a scout, not a combat ship, so you shouldn't be putting it into combat with enemy vessels anyway. Unlike the scouts for the other races, the Jikara does not have any upgrade that gives an offensive potential. It can however, capture neutral resource extractors, which is very useful.
Ravastra Skirmisher
Though technically the light frigate of the Vasari fleet, the term "light" is a bit misleading in the case of the Ravastra. It is the toughest of all the light frigates by a fair margin. Compared to a TEC Cobalt light frigate, it has several hundred more hitpoints and more armor. It is not much better in terms of offense. The Ravastra has two researchable abilities. One is regeneration, which powers down the Ravastra (making it static and defenseless) in order to achieve health regeneration. This isn't very useful, as the health regeneration rate is low. The other is Interference. This is a better upgrade, as it doubles the antimatter cost and cooldown time on abilities used by enemy ships. If micro-managed well, a small group of Ravastra can significantly handicap an enemy support cruiser or capital ship with this upgrade.
Kanrak Assailant
The Kanrak is the missile frigate of the Vasari forces. It is tough for a missile frigate, but still weak against fighters and close combat. Its missiles are notable because they are Phase Missiles, which have a chance to bypass shields. Through research, you can upgrade these missiles so that their chance to bypass shields is significant. This is very deadly against the Advent, which relies on shields for protection. The Kanrak can also be upgraded with the Discharging Missiles upgrade, which increases range and gives splash damage in return for limited mobility. This is not a bad upgrade to get, provided you don't mind micro-ing some Junsuraks and Ravastras to keep your Kanraks from getting caught with their pants down.
Karrastra Destructor
Not much to say here. They are planet-killers, and the usual rules apply. They're weak against everything except planets and they need to be well defended. They have no upgrades.
Junsurak Sentinel
The Junsurak is your typical anti-fighter frigate, except that it has Phase Missiles like the Assailant, which make it moderately more useful against enemy frigates. In addition, the Junsurak can be upgraded with Discharging Missiles just like the Ravastra, which gives it splash damage and increased range in exchange for mobility. When pitted in a straight-up slug-fest against an enemy fleet, this upgrade is very good, as it lets each Junsurak Sentinel cover more of your fleet.