I believe this should do the trick
delete vlans default
set vlans default vlan-id 1
set interfaces interface-name unit 0 family ethernet-switching vlan members default (interface-name is all interfaces you want as member of vlan 'default', vlan-id which is now 1; unit number must be 0 for family ethernet switching. This may already be in the config, so need to add.
set interface vlan unit 1 family inet <IP address/mask>
set vlans default l3-interface vlan.1
For last 2 set commands unit # must match, and best practice is to map this to vlan-id, although not required.