If your car went into limp mode VCDS will have logged a fault. There are a couple of things that it 'could' be but these are pie in the sky until you have the diagnostic check done. You ask how VCDS would have been helpful, well it will tell you the action that caused the limp mode or at least the component at fault.
The point I am trying to make is, would you like someone to say yeah go on Joe fit a new N75, would that make you feel happier? You'll go out and buy one, fit it and then come straight back here saying that it wasn't the problem. You'll be £20 or £30 worse off (plus an hours labour if you don't fit it yourself) and still none the wiser.
If the Forge N75 had failed you'd have similar symptoms but how will you know - or do you just say fcuk it, buy both and hope for the best??
Like I said above, I'm not trying to have a dig at you I'm trying to help you understand that your car is a very complicated machine and simply guessing what the fault might be is not the way to fixing your problem.
And no VCDS won't tell you the function of a component, but I gave you that above. You asked if I thought that was the fault with your car which is where I am going with this.