Sloped Zero Fuel Line on C of G chart
 

I have recently being doing a bit of flying on a Piper Seneca 2 with a VG kit. The stock airplane has a 4000 lb zero fuel limit which is a flat line from most forward C of G to most aft C of G. The VG kit comes with a W & B supplement. The zero fuel limit is now 4040 lbs at the most forward C of G to 4163 lbs at the most aft C of G. I am trying to wrap my head around how;

1) A VG kit can affect a structural limit, and

2) Why the zero fuel weight would change with C of G location

Anyone got an explanation ?

'Just guessing.... I'd expect that the VG's change the lift characteristics of the wing (usually their purpose), perhaps spanwise, as well as in total. The VG's may also change the aileron effectiveness for low speed control. So the wing bending loads could be changed, allowing more of the load to in the fuselage than originally intended by Piper, as the wing bending loads have been reduced a bit by the VG installation. Thus the VG manufacturer can claim greater payload. But, it's my guess, I don't know this VG kit.

Keep in mind that ZFW limits can get a tad involved and a bit messy, rather than being just the usual simplistic pilot training idea. There are several useful threads in PPRuNe, such as

Why have a Max Zero Fuel Mass? - PPRuNe Forums

which might be worth a browse.

Was looking on manufacturers websites and the increase in MTOW is easier explained then the change in MZFW.
One of the most comprehensive explanations is here:

https://www.avweb.com/ownership/vort...aids-or-magic/