Where do you find, or in what module do you find, Vicious weapons?
The description for a vicious weapon can be found on:
- Page 209 of the DMG
- Page 248 of the System Reference Document v5.1
- The Vicious Weapon entry on D&D Beyond
There's a text discrepancy between the three versions - the DMG and D&D Beyond cites a static figure (7, the average of 2d6) while the SRD indicates to use a roll (2d6).
When this answer was initially written (June 7 2017), D&D Beyond and the SRD matched. Sometime between then and now (June 7 2018), D&D Beyond was changed to match the printed DMG.
The description for a vicious weapon can be found on page 209 of the DMG or page 248 of the SRD.