The problem was not resolved by a common bootrec /fixmbr command issued on the Vista Recovery Console and I could not solve it using grub too (btw, my system is in dual boot with Ubuntu Intrepid Ibex).
After googling a little bit I’ve found some instructions suggesting to unhide the Vista partition, but suddenly I discovered my Vista partition was not hidden, instead it’s partition type was unknow!!!
The output of sudo fdisk -l infact was reporting a type unknow for my /dev/sda2 (my Vista partition), consequently I opened up the Gnome Partition Editor, selected my Vista Partition and changed the partition flags adding the hidden flag, suddenly the fdisk output changed reporting an Hidden HPFS/NTFS partition. Reopening the Gnome Partition Editor and removing the hidden flag restored my partition type to 7 HPFS/NTFS.
For sake of completness my partition and mbr mangling was due to my attempt to configure my Dell XPS M1330 power on buttons as this notebook has two of them: one to boot into the system and a second one to boot into Windows Media Direct. My attempt was to use the normal power up button to boot into Ubuntu and the second one to boot into Vista. More on the topic will follow if I’ll resolve the boot problems 😉