I don't have any experience with the VT2, but can comment on BIIX and Z Axis Series.
I fish a 10ft 7wt BIIX for Great Lakes steelies and it is an outstanding rod for indy nymphing. Love it for this application. I can throw smaller streamers with it very effectively. What a pleasure it is to fish this rod. The rod is capable of throwing larger ones as well, but I find that I have to really get into the rod when throwing larger flies. Is it capable, absolutely but you have to adjust the stroke. I love the sensitive tip of this rod for nymphing. With that said, there are times when I feel slightly under gunned with it when throwing multiple nymphs and streamers, the streams I fish are small compared to some rivers. Again, just adjust the stroke and I can generally get it out there. The butt of the rod is very strong, I've never felt under gunned when fighting a fish.
While I don't own a 7wt Z, I do have a 5wt and that rod is incredibly versatile. Streamers, multiple nymphs with indy, dry flies, small midges (#26)--all no problem. This is a much faster rod to me than the BIIX but with also with a soft and sensitive tip. Unless I'm going for some serious distance or throwing large streamers I don't have to make as many false casts. This is another rod that I just find a pleasure to fish with.
Both of these are light, sensitive, and capable rods although for sensitivity, I'd give the slight edge to the Winston.
Casting one is always a solid piece of advice if that's possible. I also agree with Frank's assessments about people's opinions of various rods and more important within a given line that certain rods within a specific line excel more so than others. I think you should be asking yourself if you like slower action rods or faster action. Think about what types of flies you expect to throw on this rod and the size of the rivers. This may help you hone in on your decision.
My disclaimer is that I am not a guide, casting expert or instructor, just a guy that loves to fish and loves gear
Good luck, happy shopping and let us know what you end up with.