The affix ṭhañ comes after the name of an explainable work called after a ṛṣi in the senses of 'occurring therein' or 'commentary thereon' when it refers to an adhyāya only.,
The word ऋषि is the name of प्रवर and those Rishis are only meant whose names are in the Pravara list. The word so formed means always an Adhyaya or Chapter of the work. The phrase व्याख्यातव्यनाम्नः is understood in this sutra, hence the word ऋषि means \a work the author of which is a Rishi\.
Thus वसिष्ठस्य व्याख्यानस्तत्र भवो वा = वासिष्ठिकोऽध्यायः \Vasishthika- a chapter containing commentary on Vashishtha\. वैश्वामित्रिकः 'a chapter of commentary on Visvamitra'.
Why do we say meaning 'chapter'? Observe वासिष्ठिकी ऋक् ॥,