Microsoft has a FAQ: Visual Studio and Windows Vista (http://msdn2.microsoft.com/en-us/vstudio/aa948854.aspx) where it is stated that Visual Studio .NET 2002/2003 will not be supported on Windows Vista, and that only VS 2005 will be supported with the SP1 and a (post-SP1) Vista Support Update. I have some comments about this:
The first one is that although .NET Framework 1.x applications will be supported, since an add-in is intended to be hosted inside a Visual Studio IDE, and since VS.NET 2002/2003 will not be supported, there is no need to test an add-in for those IDEs on Windows Vista.
Second, the VS 2005 SP1 will ship soon (maybe December 2006, maybe January 2007) and the Vista Support Update will be released “in the first quarter of 2007” but SP1 will not wait until then because “Delivering great product quality is something that we have always aimed for. We wanted to deliver the update to Visual Studio 2005 as soon as possible to our existing customers”. Given the long delay of the SP1 of previous IDES (VS.NET 2002/2003), it is quite shocking that VS 2005 SP1 can not wait a couple of months and include the Vista Support, because to run VS 2005 on Vista you will have to go through three installations, and it can be a mess.