The minimum requirement for cluster is 2 SRX of same hardware and running same Junos.
The process called JSRPD takes care of synchronizing the config, sessions, VPNs and a whole lot more.
If that's not posible in your setup, then VRRP may be the solution for you.
Config: http://kb.juniper.net/KB24681
You should however note that VRRP is not stateful and all TCP sessions would have to be re-established.
The benefits of having a proper cluster far outweigh VRRP, in my experience..
Regards,
Srinath