I spent years at Walmart Corporate and they had a vendor management team in the hundreds to support their IT organization.  

If you're a smaller company, you obviously can't do that. If that's the case, where do you start? It's not practical or necessary to learn every last detail about how to manage a vendor, so focus on what's most important to you.

What defines success for your organization with a vendor? Are you looking to reduce risk? Keep costs down? Better soft skills for having difficult conversations? Do you want to closely monitor to ensure they're meeting their KPIs?  

A healthy approach is to understand all of the aspects of vendor management topically, and if you're going to drill down into a specific subject areas, choose the one that has the biggest impact for you and your company to start.  

Remember, deciding to choose a vendor means you're reliant upon someone else outside of your organization to hit organizational goals. That's a big deal. Take this kind of training seriously and you will achieve success with that vendor much sooner.