IPSec VPN using alternative IP

I'm trying to create a new Site2site using an IP which is within the subnet for the external interface. For example:

External Interface

Reth5.10

10.1.1.1/24

But I'd like the local-address for the IKE gateway to be 10.1.1.2. I've managed this previously by creating a new vlan as below but stating within the IKE gateway to use external-interface of Reth5.10 and local-adress of 10.1.1.2:

Reth5.2

10.1.1.2/32

However, this doesn't appear to be working. Any thoughts on how to accomplish this? Currently running 15.1X49-D180.2 on SRX4100 cluster.