Do I file an extension? I funded a tIRA on 4/11/19 to convert to a roth ira, vanguard makes me wait until 4/18/19 for the funds to settle to convert to a Roth IRA.

No - you don't have to file an extension, you can just complete your return as if the conversion has taken place.

Because the Roth conversion will have occurred in 2019, the Roth conversion will be reportable on your 2019 tax return, not on your 2018 tax return.  Only the traditional IRA contribution made for 2018 is reportable on your 2018 tax return.  (Just make sure that Vanguard is treating the traditional IRA contribution as a contribution for 2018.)