The code-G Form 1099-R with the IRA/SEP/SIMPLE box marked indicates that the money from the original 403(b) was rolled over put into a traditional IRA (a rollover IRA), not a traditional 401(k). The traditional IRA was subsequently rolled over to the new 403(b) and that's what this Form 1099-R from Vanguard is reporting. This Form 1099-R must have a zero in box 2a. You must indicate that the distribution was not rolled over to any kind of Roth account. The gross amount from this Form 1099-R should appear on Form 1040 line 4a but be excluded from the taxable amount on line 4b. The word ROLLOVER should appear next to line 4b. Such a rollover is required to appear on your tax return in this way.
If also done in 2020, the rollover from the original 403(b) to the traditional IRA would have been reported on a different Form 1099-R with code G and the IRA/SEP/SIMPLE box not marked and should have a zero in box 2a. The gross amount of this distribution should appear on Form 1040 line 5a but be excluded from the taxable amount on line 5b, with the word ROLLOVER next to line 5b.
