Currently, the ability to create/define a role is not exposed through SOAP API. Feel free to enter enhancement request in the bugzilla.

For existing roles, you can view and set role membership on a given binder by using the following three operations - binder_getFunctionMembership, binder_setFunctionMembership, and binder_setFunctionMembershipInherited. Unfortunately, there is no binder_getFunctionMembershipInherited operation though. We have a bug request for this (bug #626658). By the way, "function" is internal name for "role".

You can not rename user (i.e., changing user's login name) via SOAP API. As a matter of fact, renaming of a user is not supported in the current version of Vibe.

Most of the operations manipulating workspaces and folders begin with binder_.

I'm curious about your "role definition" request, do you really want to define new roles at the site level? Or are you talking about adding and populating roles and role membership on a binder? The former is done at the site admin level. We don't expect people to be adding or changing role definitions with any frequency. They should be defined once and then left alone. The latter is a normal part of setting up the access controls for a folder. As I understand it there are calls for all of those functions (except as Jong noted about getting the inherited rights which you could do by working your way up the parent folder chain using the calls that do exist).