I didn’t think there was much that Vista users could do about this at the time of your post. However, discussion in the extended kernel thread on November 7 revealed that .NET 4.6.2 now officially supports Server 2008 SP2! (4.6.0 was formerly the highest officially supported version for 2008, and it remains so for Vista.) No workaround is needed to run the latest 4.6.2 installers, however they have sha-256 digital signatures now.
Edit: I just found that this was mentioned at .NET Framework 4.5.2, 4.6, 4.6.1 end of support FAQ:
I use .NET Framework 4.5.2, 4.6, or 4.6.1 on Windows Server 2008. What should I do?
Previously .NET Framework 4.6.1 and earlier versions supported Windows Server 2008 but .NET Framework 4.6.2 and later versions did not. As part of this end of support update .NET Framework 4.6.2 has been updated to include Windows Server 2008 as a supported platform. Customers using .NET Framework 4.5.2, 4.6, or 4.6.1 on Windows Server 2008 should plan to move up to .NET Framework 4.6.2.