Vouch for is a phrase that means to provide assurance or guarantee of something. The crossword solutions for this phrase are SPONSOR (7 letters), SWEARTO (7 letters), and ATTESTTO (8 letters). SPONSOR implies providing support or backing for someone or something, while swear to and attest to both involve making a solemn promise or statement of truth. All three words can be used to vouch for something, as they all involve providing assurance or guarantee of something.