I have no doubt that the holster will hold up over the long haul. Due to the large number of people who wear them, and the excellent feedback I have heard, I am confident that it will hold up. That is not my question. I am simply trying to understand what the pros and cons of having the slit are. So far, I've got 2 people on this thread telling me it's for better tuckability...but as you point out, I don't see how it would help tuckability when there's no back slit and when it comes with Kydex clips anyway. Now you're telling me it's not for tuckability, but you didn't provide an alternative explanation...so I hope that when more people respond I can get to some kind of logical consensus.
I don't have a VM II so I can't answer your question about the front wing flexing. The holster has been out for quite a few years though and MANY, MANY people have (as shown above) more than one in their collections. Those guys who wear them all day every day should certainly be able to report on how the holster holds up over the long haul.