You're asking buy versus build I assume. From what you're describing, I'd buy, or, more likely, pay for a service. Vendor management and Customer Relation Managers (CRM) systems are pretty similar, so you're going to see a lot of crossover. Here's a list of top vendor management software. I'd pick the one from this list that best fits your needs.
Otherwise, you just have to build a relational data store and a web front end. None of that is hard, unless you don't have the skills within your org. That's why I'd go with getting a service instead of building yourself.