Well, this is where things get confusing. I don't retune a helicopter to carry a payload, I expect it to be more sluggish because it's carrying a load. If that load the normal part of the helicopter, then tune for it. If the helicopter carries varying loads like mine normally do, then you can't tune for it.
The PID loop is a proportional-integral-derrivative loop. So if there's an error in attitude, the P gain provides the proportional, or near-term correction to get to the target. The I gain integrates the error and provides the long-term correction needed to make it hold zero error. The derrivative acts like a shock absorber. If the P tends to over-shoot the D will dampen it and prevent the over-shoot. Using some D gain tends to soften the P a bit, but some people can't use any D gain because it causes oscialltion problems. I can and it makes the P gain more accurate in what it does.
As far as the VFF, as I stated before I fly my flybars with just VFF. The PID's are turned completely off. While there is some disagreement on what a flybar actually does or how it works, it's really that simple. Just turn off the rate PID loop and use VFF and turn it up until you get the control response you want, and it holds actually better in the wind than the DFC helicopter does in Loiter.
So I think the flybar does the same thing as the rate PID loop. For FBL just set the rate PID loop so it flies like a flybar in Acro and you got it. The trick is arriving at the settings to make it do that. And every helicopter is different.
When I tuned my Trex 500 I inititally went for the high P values in the rate controller and I got it up to .19. But after I flew it for awhile it just didn't have that flybar "feel" and stability and could be pretty easily upset in flight if I started pushing the speed. So it was way over-tuned. I didn't do any "formal" PID tuning after that point. I just kept turning down the P gain, turning up the I gain, and turning up the VFF on subsequent flights until I got it to where it feels more like a flybar helicopter. And those settings are what I posted above. Except the VFF is turned down just a tad right now for my wife to fly it, so it's not quite so quick on response.
So I tried it on an auto flight with those settings and it flew the best its ever flown in Auto. Not as stable as my flybars. But it's really close. Some people fly I gain up to .5 in their helicopters. My Trex 500 doesn't need that much. And I found out I can reduce the P gain and get rid of a lot of the quirkiness and jittery characteristics it had, and lean heavier on the VFF and it's lot smoother.
So what I ended up with in my FBL heli is first trying it the "textbook" method and got a fairly decent flying helicopter that I just wasn't happy with. It reminded me of flying a full-blown, sharply tuned 3D heli in scale flight. It just wasn't smooth. So then I went the trial and error method to find that flybar "feel" that I really like. And that's what I ended up with. What's funny is that when I first tuning that thing I had it about where I got it now and tried it on an Auto flight and it was horrible. So I went with the whole tuning adventure. Ultimately I ended up basically back where I started, except I got WAAY more VFF now than I had the first time, and that made all the difference.
I don't know if yours will tune that way or not. But it gives you an idea of what to look for in how you want the heli to feel. Do you like the sharp 3D type tuning? Or do you like flybar? The trick for me was in finding the right settings to get the flybar with good Loiter/Auto performance.