Box 2a equal to box 1 with box 2b Taxable amount not determined is required reporting for any regular distribution from a traditional IRA, including those for a conversion to a Roth IRA, because it's impossible for any IRA custodian, including Vanguard, to determine the taxable amount of the distribution because they have no knowledge of what other traditional IRA accounts you might have. They also have no way to know how much basis you have in nondeductible traditional IRA contributions; that's between you and the IRS. The taxable amount is not known until you prepare Form 8606.
The code 2 in box indicates that they know the distribution is not subject to penalty because they know that the entire amount was converted to a Roth IRA (since they performed the transactions), but that's all they know (other than the gross amount).
