It appears that the pipeline has changed between webforms and MVC. The Success pipeline for MVC is <wffm.success>
and can be found in Sitecore.MvcForms.config. Through this you have access to the FormModel
public class SuccessRedirect : FormProcessorBase<IFormModel>
{
public override void Process(FormProcessorArgs<IFormModel> args)
{
Assert.ArgumentNotNull((object)args, "args");
var model = args.Model as FormModel;
if (model == null)
return;
model.SuccessRedirectUrl = model.SuccessRedirectUrl + "?val=" + model.Results[0].Value;
}
}
Check out http://ggullentops.blogspot.com.au/2016/07/sitecore-wffm-act-on-success.html for more information and some options on customising post-success actions.