If you'll let me play devil's advocate for a second: are you sure it's the impostor syndrome? Is there any chance that you're (with the best of intentions) misr

If you'll let me play devil's advocate for a second: are you sure it's the impostor syndrome? Is there any chance that you're (with the best of intentions) misrepresenting your knowledge of the vendor to the client? That would certainly induce a fear of failure and being 'found out'. Do you feel your clients understand that while you have a great track record and are knowledgeable in the problem domain, you don't have experience with their specific vendor? If not, maybe explaining that and framing it as a project risk would help with the fear?