Is it correct that vouch is only to claim something true without proof, while attest is to prove something true?
I feel the word 'vouch' does not necessarily mean the truth 'without proof'. It can also mean providing 'supporting evidence'. For instance, if you commit that you'll get an 'A' grade in an examination and then if you get it, I say "Tim vouched his words by getting an 'A' grade." On the other hand the word 'attest' is more reliable, firmer and authentic. You 'attest' the documents and signatures. You also confirm the event using the word; for instance, "The weakness attested to his dehydration."