Yogavasishtam is doubtlessly one of the greatest works on Vedanta.

The great treatise yogavasishtam even though not part of valmikiramayanam is

a prime philosophical work of supreme value and fame comprises of spiritual lessons imparted by sage Vasishta to Rama.

The authorship of yogavasishtam too is attributed to  Maharshi Valmiki.

True, Visvamitra trained Rama substantially in archery ..

and later on Agastya too did his mite ..

However, it was sage Vasishta who was the guru and teacher for the four boys following the tradition from the earlier generations.

And Yogavasishtam is doubtlessly one of the greatest works on Vedanta.. finding its place of honour along with Upanishads, Geetha, Brahmasutram, ( called the prasthaanatrayam)