With IPSec VPN, you can use the "crypto map local-address" command to use a different interface (for example: using loopback interface, then you can assign the spare public ip address to your loopback interface).
Here is the command for your reference:
Under the webvpn gateway section of the configuration, there is an option to configure ip address:
webvpn gateway ssl-gw
Here is the configuration guide for your reference:
Hope that helps.