The affix ṭhak comes after vāc when it means the purport of the message spoken by another.,
The word व्याहृत means 'declared, expressed'. That which was spoken formerly by another, and which is carried by a third party as a 'tiding' or 'news', is called व्याहृतार्थ ॥ Thus वाचिकं कथयति 'he relates the verbal message'. वाचिकं ऋद्धे ॥ Some say it applies to written message also.
Why do we say \when it means a verbal message ?\ Observe, मधुरा वाक् देवदत्तस्य 'the speech of Devadatta is sweet'.,