Hurrah, I finally suceeded to upgrade the Bootloader, but I was nearly to failed.
I bought a cheap clone of the usb avr cable USPasp.
With it when using avrdude 6.1 under win7 I had write errors on lock bits which ends to get lock bits = 0x00 000 which forbids any further writing on the chip.
So the Vems team sent to me a bundle of fw and bootloader to flash the chip after an erase.
After the flash, the verify failed, again and again. To be sure of grounds I powered the chip from the vcc of the cable instead of the vcc of the board. Failed again and I was ready to buy the USBISP cable from the Vemsshop.
As a final try, I downloaded the old WinAVR_20100110 which contains the
(bin folder) suspecting that the fw of the clone USBasp was quite old as well.
Success, no writing error anymore, I flashed, set the lock bits 0x3c, and downloaded the new bootloader through VT.
Thanks Dave for your help
Philippe