Have you checked the 1099B(s) loaded into TT to see whether it duplicated the import somehow, if so delete the dupe. Line 1a/8a is usually populated by "sales summary" 1099Bs in TT without any sort of adjustment code, which do not generate 8949 entries.
On desktop you would go to Wages & Income / I'll Choose What I Work on / Investment Income (1099-B) / Stocks etc. Not sure online but presumably equivalent function to review your 1099Bs.
If there is no obvious dupe 1099B you can try deleting the Vanguard 1099B that was input and see if that cleans the Schedule D back up, then either re-import or input the 1099B manually.
For manual input, if the 1099 is straightforward without any wash sales/adjustments then you can just input it manually as a sales summary which will go on Line 1a/8a. If you have adjustments (Box 1f AMD, Box 1g Wash sale etc) or noncovered securities categories those are best input as "one by one" with the details and input the balance of your 1099B as a separate 1099B as "sales summary" to avoid a requirement to mail your 1099B to IRS with the details.
