[Uncreative]

I actually like to entertain the thought that the Vajra have a hive-mind similar to the Formics (buggers) in the Ender's Game novels.

Spoiler for Ender's Game ending and series info:

The Formic species consists of hive-minded colonies that are directed by queens by a telepatic means of communication that stretches over light-years. If a queen dies, all the drones under her control lose their intelligence and ability to function immediately. Formic queens communicate instantaneously and can even do so with other species, although this is more difficult.

Simply put, the Vajra are just some space-faring pest that the protoculture stepped on or wiped off their windshields. They exterminated their queens long ago and now the mindless workers simply drift in space until humans came along and were able to "mimic" the presence of a queen through song power. This presence awakened them and they are simply trying to find and "protect" their queen.