If the form won't submit, one of the fields might be failing validation. I would expect an error message to appear directly above the field that is not validating.
For the attachment path, it cannot use the path specified in the vB install's own settings, because that is the path relative to the vB file system. If the XF install is on a different server, it is unlikely that the path to the attachments is relevant to the XF server. You must specify an attachment path that exists on your XF server, and the vB attachment files must be located there.