Emphasizes your confidence in someone's abilities or character, often based on experience. Don't worry, I can vouch for her; she's excellent with numbers.
Pro Tip 2/3
Personal Assurance
Used when offering your personal guarantee on something, showing personal accountability. I'll vouch for the laptop's quality, I've been using it for a year.
Pro Tip 3/3
Not Legally Binding
While it shows strong belief, 'vouch for' is not a legal guarantee or proof. I can vouch for his honesty, but the court will need more evidence.