A more thorough explanation:

VOUCH OVER

vouch over, verb. To bring someone to court in place of oneself.

1. The defendant was unable to attend the hearing, so he vouched over his lawyer to represent him.

2. The witness vouched over her friend to testify on her behalf.

When someone vouches over another person, they are essentially asking that person to take their place in court. This can happen for a variety of reasons, such as illness or scheduling conflicts. In the examples provided, both the defendant and the witness were unable to attend court themselves, so they vouched over someone else to represent them or testify on their behalf.