Vouchers are for pre paying 2018 taxes. If you selected Yes you want to set up a payment plan at the end of your tax return, then the IRS will be contacting you by mail soon. They will evaluate your request for a payment plan and then decide how much your monthly plan will be.
For your further information on setting up a repayment/taxes due for 2017 plan, please click the link below to the IRS' web page "Additional Information on Payment Plans:"
https://www.irs.gov/payments/payment-plans-installment-agreements