If you are trying to configure SoftNAS virtual IP based HA and are confused as to what you can use for a VIP their is a simple answer. The VIP IP can be any IP address that is not within the overall CIDR of the VPC. For example if your VPC CIDR is set to 10.0.0.0/16 any IP address that does not fall within that CIDR will work as a VIP address.
To ensure your IP is outside the CIDR block is simple. Ensure that the third IP starts with a different octet. Starting with a different octet means that if your VPC range is 172.16.0.0/16, the VIP must not start with 172, it must start with something else, for example: 10 or 12 or 175.