Did backdoor Roth IRA, but didn't receive 1099-R form; timeline from Vanguard is after 4/15. Guess at 1099-R info 4 Turbotax (& possibly amend), vs ask for extension?

You can use your best guess to estimate whether you owe any taxes for 2019, and then file an extension. You should not file your return without the actual 1099-R information. 

If the distribution converted to Roth was obtained in 2019, the traditional IRA custodian was required to send you a 2019 Form 1099-R.  If this distribution occurred in 2020, the Roth conversion is reportable on your 2020 tax return not on your 2019 tax return, and the only thing reportable on your 2019 tax return would be the nondeductible traditional IRA contribution made for 2019.