TL;DR:

As the title says, I'm guessing there's a reason development went past .NET Framework 4.0.
I've looked at porting it myself, but time and effort are something I cannot afford right now, and I really struggle with low-level stuff like this.

For nostalgia reasons, I would like to play older games but with a newer sim wheel, which puts me in this predicament:

Option 1: Buy an old wheel, which I would need to explain to my wife.
Option 2: If your efforts can be bought, I'll make a weak offer for the price of an old wheel ($128.63 USD locally).
It's easier to hide than a physical object in the man cave.
You'll also help out the boys in the retro sim community if you care about that.

Also, thanks for what you are doing, keeping vJoy semi alive. It seems like this is currently still the latest version I could find.