The problem with this approach is that the device needs be reset to enable HV mode. When you supply it only from the battery, it's all fine, and when VBUS is applied, the device will be reset since REG0 (VDDH) is not enabled yet and VDD will be lost. So it powers up fine again with VBUS as the PS. But when you remove the USB connection, REG0 will be powered backwards and the device still thinks it's in HV mode. That may have unpredictable consequences.
A better way to do this is to use a external regulator with a series diode to VDD and the battery, also with a series diode to VDD. USB 5V can be connected to VBUS to detect that the USB is connected. This way you don't use REG0 at all and will avoid having to reset the device.
The problem with this approach is that the device needs be reset to enable HV mode. When you supply it only from the battery, it's all fine, and when VBUS is applied, the device will be reset since REG0 (VDDH) is not enabled yet and VDD will be lost. So it powers up fine again with VBUS as the PS. But when you remove the USB connection, REG0 will be powered backwards and the device still thinks it's in HV mode. That may have unpredictable consequences.
A better way to do this is to use a external regulator with a series diode to VDD and the battery, also with a series diode to VDD. USB 5V can be connected to VBUS to detect that the USB is connected. This way you don't use REG0 at all and will avoid having to reset the device.
