Many people don't know that the sizeable Target breach occurred because of a third party vendor, an HVAC company's lack of security. This article from CSO Online gives you advice on how to approach relationships with third party vendors.
David Baker, vice president of operations at Bugcrowd, said "The rule of thumb most CSOs live by is that you only use a third party if they do something better than you. So whether that’s delivering a package or managing your data center, if an outsourced third party does it better, it makes sense to use them. This extends to security... If you use a third party and want to avoid something like what happened with Target, you need to have a process by which you select those third parties, and a big part of that criteria should be security. Security has to be something you can measure that they do better than you.”
Essentially, each third party vendor you work with represents a back door into your organizations network. Read more on what you should be aware of and steps you can take to be more informed and prepared here.