How are soft credits used for membership payments in webform?
Download the latest -dev of webform_civicrm module. We recently merged in some code that should let you do this (explicity add the soft credit).

This is the commit:
https://github.com/colemanw/webform_civicrm/commit/bc7e0eee2a39ca0c35e34109d3ff05ee2e26b8d3