There is a very old Indoeuropean root which in the vedic language had forms in vas/uS which meant to shine, be bright and good. vasu from that meant very good or very rich, riches, or desirable objects. vasiSTha, sometimes vashiSTha, is the superlative of that, though rarely connected with a different root vas which meant to love in the very carnal sense.
vashiSTha was the name of a very famous holy man who appears in mythology very often. For example, he was the owner of the cow nandinI, the daughter of surabhi, who was called kAmadhenu, or the wish-granting cow. Through her, he could get any thing one desired, and was, therefore vashiSTha. The vedic mythology puts him on opposite sides to the other major sage visvAmitra, so much so that many have taken the descriptions as allegorical of two branches of vedic people fighting for supremacy in India, the resident vashiSThas and the approaching visvAmitras. He is said to be the son of Mitra and Varuna by the apsarAs and is the family priest of various major lines of kings. When the big bear was renamed as seven sages, he was counted as one of them; he is also mentioned as a progenitor in the myths that peopled the world.
The name has been common throughout the ages, and in modern Indian languages, has changed pronounciation slightly.