If you are licensed properly, you can give permissions for your vendors to access a form library as part of your Sharepoint deployment. Depending on what version you have, this licensing could be reletively inexpensive. So you would provision a site for form deployment, publish your form there and direct your outside vendors to this site. You would have to give them logon credentials, but they would be able to sign in, fill out your form and submit it with no issue.