If your nanny has chosen to self isolate, then you would pay SSP.
The Job Retention Scheme is designed to continue pay part of employees’ salary for those employees that would otherwise have been laid off during this crisis. You are not considering laying off your nanny... if they were not self isolating then they would be coming in to work.
So in your particular situation I think you only pay SSP.
With regard to childcare vouchers, I expect those can be stockpiled for a while, so can be used in the future. Your voucher provider may be able to tell you for how long you can stockpile vouchers before they remove you from the voucher scheme.
You might at some point use a different childcare provider, so you may be able to use your childcare vouchers with them.
You may be able to pay SSP with childcare vouchers... vouchers are just a payment method, so as long as your nanny remains a registered/approved childcare provider then I think you can use vouchers to pay a Statutory payment.
This is all so much of an unknown at the moment. In your particular situation I think the key is that your nanny has chosen not to come in to work, rather than you deciding that there is no work for them to do and thus they are being laid-off.