so second why do i loose boot vector?
Answered in my previous post.
and if a buy an atmel at89c51rc2 would i face the same problem???
No, your problems will be different.

Changing chips in the middle of the stream is rarely a good idea, while you might get rid of a problem, you usually will find out later that you picked up another. You can hope that the errata sheets (check them for the P... and the T...) list everything there is abnormal about the chips, but who knows, you might be the first to find out.

Erik