Current versions of Visual WebGui check specifically for the existance of the Visual Studio version they are intended for. This means that installing the closest NET version, NET 4.0 for VS2010, refuses to install without Visual Studio 2010 installed.
Although you could install it, you would not have any integrations, templates or designer support within Visual Studio 2011. This means there will have to be a VS2011 Visual WebGui release, and I have requested more information on our plans to support that.
We will update the community as soon as there are any official plans for this to happen.