There is a more painful question. No corporation can ignore the dangerous behavior of vendor employees, or their lack of training. The duty to provide a safe workplace is not dependent on who provides the paycheck. To the degree you create a hazard, control it, or have the responsibility for correcting it, the duty to initiate safe practices is yours. In some situations, federal codes forbid you to delegate safety responsibility at all.
Corporate responsibilities begin with the choice of the tools for each job.
If you use a repetitive process, conduct a survey of climbing jobs. Determine what type of ladder to use, and if a ladder is the right tool for the job. A survey includes an assessment of the need for personal restraint systems. It also includes the engineering of tie-off points at an elevated position when you need them. This can be a shared responsibility.