Please check MB91F460 TRM. In chapter 53 (Fixed Mode-Reset Vector) it is mentioned that these two vectors are independent from the flash and cannot be erased. This means that in internal vector mode (MD[2:0]), after reset the BootROM is always executed at address 0xBFF8.
By default the user program entry address is set to Flash address 0xF4000. This can be changed by the boot security vectors BSV1 (0x148004) and BSV2 (0x14800C) in order to set a new entry address and to skip the internal bootloader managed by the BootRom (see also figure 53.2-2).
When I deleted the sector that contains Fixed Mode Vector and Fixed Reset Vector, and I tried write the new data in this sector, the processor stuck off.
Since then, bootROM never started, I don't get ACK response from bootROM.
As mentioned, FMV cannot be erased. What does they mean with they don’t get ACK response from the BootROM? The BootROM is providing a simple internal bootloader in case BSV1/BSV2 have no valid data programmed. If BSV1/BSV2 have valid data, the internal bootloader is skipped and no communication can be done via the internal bootloader. In that case a user software must already be programmed which is communicating via an interface (like a CAN bootloader) in order to re-program or to erase the Flash. If this software is not present in the Flash, the Flash can only be erased by a parallel Flash programmer