First let me state this is for Windows 7 platform and not Vista, however, there was not an option for Windows 7 so perhaps that’s the answer - Lotus Notes 8.0.1 is not supported on Windows 7?
The issue is the mailto functionality - it cannot be configured. Windows 7 has removed the default mail client and all articles assume the user will be using Windows Mail (downloaded through Live Essentials) or a third party client like Thunderbird etc.
What we have discovered is that windows updates always sets this back.
You can go into your notes client and tell it that it should check to see if it is the default mail client all you want. You also get that prompt when reinstalling notes. But the next time windwos updates come around - woosh, there it goes back to being windows mail or whatever.
I’m unsure of the issue you are experienced, but do know that Windows 7 is Supported on 8.5.1 FP1.
Also, for reference, I have included a Technote below that outlined the Supported Platforms for Notes Clients and Domino Servers from Release 6.x - 8.5.1: