There are multiple definitions - click on the translation that matches your context.

保证

动词

full

vouch for

verb

Definitions of vouch for

to say that a someone has the qualities necessary for a particular task or position

My client was then driven to his hotel where staff were able to vouch for him.

to state that you believe something is correct or true; to say that you take responsibility for something

CBS has admitted that it cannot vouch for the authenticity of documents used to support the story.