Are we speaking of the vector located in 0xfff0 ? if yes, this means that I should add in my code a line to set the vector pointer at this location ?

Am I right ?
No, The P89.. has two separate registers which both must be set correctly to allow ISP. They are the Boot Vector and the Status Byte. The Status Byte is automatically changed when you do the funny stuff with signal levels at reset, the Boot Vector must be programmed. When you access ISP, the Boot Vector register must point to the ISP code. It is programmable so that it can be modified if you write your own ISP code instead of relying on the ROM ISP loader. I personally think this feature is stupid, if anyone writes his own ISP loader, thet would not be activated by funny stuff with signal levels at reset, but by a command in the regular process.

Erik