Because both Freespaces are played from a terran perspective, :V: never think about Weapons for the other races, because there are not playable in official campaigns anyway.
That it was never intended to fly vasudan craft is also ascertainable that some ships do not have aligned primaries, there were too far apart etc.. Shivan crafts are even worse.
In FreeSpace 2 was a different story, because of the missions on board the Psamtik and Memphis. But even there the Mekhu-HL7 was the only vasudan weapon. But at least the vasudan crafts did have aligned primary guns.
For Campaigns that was released before FreeSpace 2:
Cardinal Spear: Vega featured a few weapons for the vasudans, even only as enemies.
In Awakenings you were able to fly vasudan crafts in a few missions, but only with terran weaponry.
I remember also a campaign as a Vasudan against the Hammer of Light, 'Battle for Altair' maybe? Afaik it was also only with terran weaponry.
There are also only few campaigns where you play as a vasudan that were released after FS2 or SCP. Actually i do only know Shrouding the Light and the mentioned Vasudan Imperium that featured both vasudan weaponry.