We are looking for a good talent assessment vendor/resource for Hi-Po and newly promoted employees. If possible, we would like to be able to customize to include the leadership competencies we have established as our internal leadership essentials. Who is using internal assessments, and how are you using them? What vendors do you recommend investigating? 

We utilise Korn Ferry for executive/senior development. We also utilise 360 assessments for development and partner with CCL who have customised our 360 based on our leadership competencies

I would recommend investigating Korn Ferry.  We use Korn Ferry tools to assess potential, leadership competencies, the climate created and leadership style.  I find their suite of tools to be quite comprehensive.  We have also utilized Korn Ferry to Develop a very comprehensive "success profile" that outlines the accountabilities and requirements for each stratum of work in the organization.  This has enabled the identification of high potentials and it also supports more comprehensive development planning.  We also have customized leadership competencies, but I would caution against this.  Korn Ferry and other providers often have a suite of leadership competencies that are based on comprehensive research (and often not altogether different than the internal "customized" competencies).  This means the competencies are less introspective.  And, it is much easier to integrate assessments and succession planning programming when the required competencies are based extensive research about what constitutes ideal leadership.

We are using Talogy (formerly PSI) and DDI assessments for various purposes.  We use pre-employment assessments for our hourly workforce to ensure fit and proper qualification.  We use other assessments for development purposes.  Leaders (mostly hi-pos) take a particular assessment during a leadership development program.  They use the results to leverage strengths and work on opportunities areas, then report out at the end of the program.  There are several good vendors out there, depending on the need.  Both Talogy and DDI have excellent leadership assessments that may fit your needs.  Not sure on customization, but there would definitely be an opportunity for competency mapping.  

We have used Korn Ferry which provides data driven insights for leadership assessments helping in talent decisions and succession planning.

I would also recommend MDA Leadership Consulting here; strong in potential measurement both for identification of up and coming leaders and how to develop them once identified. They do a great job aligning to pre-existing competencies.