In 2020 I converted part of a traditional IRA into a Roth IRA. The distribution is shown on line 4b and taxes were paid on the entire distribution, however, form 8606 showing the distribution was converted to a Roth IRA was not attached. Vanguard did file form 5498 showing the conversion. Do I need to file a corrected 1040 for 2020?
No, you don't need to file the 2020 Form 1040-X since it seems that the taxable part was correctly shown on line 4b.
You can file the 2020 Form 8606 by itself.
Technically you can be fined $50 for failing to file the form on time, but the penalty can be waived if you have reasonable cause. Please see Relief for Reasonable Cause for additional information.
No, you don't need to file the 2020 Form 1040-X since it seems that the taxable part was correctly shown on line 4b.
You can file the 2020 Form 8606 by itself.
Technically you can be fined $50 for failing to file the form on time, but the penalty can be waived if you have reasonable cause. Please see Relief for Reasonable Cause for additional information.