According to the Second Life wiki, the answer is no.
The wiki page in question, last updated on October 2, still says Vista is not supported:
[…] Officially at this time (2007-08-27), not yet […]
Second Life actually works fine running on Vista. That’s my experience, and that of many others, although much depends on your particular hardware configuration especially the type of graphics card you have and the version of the graphics driver you’re running.
But is it officially supported by Second Life’s developer, Linden Lab?
In spite of statements to the contrary like the one above on the Second Life wiki and elsewhere, it appears that Windows Vista is now officially supported.
The Second Life system requirements spec now says this:
Note the text in the second and in the last bullets points.
I can’t find any other information about Vista support anywhere on any Second Life site more recent than a post dated July 20 in the Second Life blog, which says:
[…] We’re in the process of validating our support of Vista. We’ve finished a couple of rounds of testing. We’re now pulling together our findings and laying the internal groundwork for formal support of Vista.
That seemed reasonably clear when the system requirements page still had no mention of Vista in the requirements spec.
Now that it does, it might be helpful to everyone if Linden Lab would be a bit more consistent in their online communication.
So is it supported? Well, it looks like it. Unless you read the contradictory words.