The
official W-9 form from the IRS has one combined option for "Individual/Sole Proprietor". There is no difference to the IRS for tax identification purposes. The Form W-9 is just the official method used to give Vanguard your Taxpayer Identification Number. If you check off "Individual", you give them your Social Security Number; if you check off "Sole Proprietor", you give them either your Employer ID Number (if you have one) or your SSN, at your option.
Since the SEP-IRA is a business-based retirement account and you are filing as a pro, I'd go with Sole Proprietor and use your Social Security Number.