Unfortunately, I haven't solved the problem yet. [:@]
The Dual Gigabit Network Adapters are recognized by Vista as "10/100/1000" but in the list of "Speed/Duplex" settings there are no items related to Gigabit (1000 Mb FD nor 1000 Mb HD). So, under Vista, the two Network Adapters are working as two normal 10/100 ports, causing all my problems with Vienna Ensemble 3.
The sad thing is that the last driver update from NVidia for Vista 64 (15.25) doesn't help at all. Even though NVidia released that update on March 16th, 2009 and this issue is well known in the NVidia forum, many people are still suffering from this absurd problem.
The only thing remains to do for me is to change the Motherboard, as I don't have any further room to add a separate PCI NIC card.
At the end of this story, I can give you a suggestion: don't trust NVidia!!!
Luigi.