Does anyone know how to get the 8820 with ATT firmware (latest) to have the Wifi preferred option in Manage Connections --> Mobile Network Options?

Apparently T-mo 8820's have this option but my ATT does not. Would reflashing it with the same firmware, minus the vendors.xml enable this option?

OR, would it be possible to flash my 8820 with t-mo firmware that has this option? (link anyone?).

OR, does the new singtel latest firmware that has english have this option?

The Wifi Preferred option is for phone over wifi (UMA.) That setting doesn't control browsing over the wifi. Since AT&T doesn't have UMA you will likely not get that option at all, even with different OS.