viper-mouse-insert-key

viper-mouse-insert-key is a variable defined in `viper-mous.el'.
Its value is

(meta shift 2)

Documentation:
Key used to click-insert in Viper.
Must be a list that specifies the mouse button and modifiers.
The supported modifiers are `meta', `shift', and `control'.
For instance, `(meta shift 2)' means that holding the meta and shift keys
down, and clicking on a word with mouse button 2, will insert that word
at the cursor in the buffer that was current just before the click.
This buffer may be different from the one where the click occurred.

You can customize this variable.