Right now, I can foresee two or three set backs; one of them is fairly small, and the other two I still need to poke around...
First, the small one is the user integration. OSC have their 2.2 Milestone 2's database structure listed in a PDF file bundled with the download. From what I can see, there will need to be a few custom fields added to vB's db, and a new registration form made to get all the info into the right tables.
Second, the slightly bigger one is the looks and feel of vB. Right now, I am not sure if the vB looks and feel can be carried over as I've had my vB license for lesser than a month and I have no idea how to clone the looks and feel yet; this could be a set back that I'll have to look at / address later.
I have yet to look at OSC's purchase system, I wonder if it would be very difficult to add a few SQL calls around them to check for user group and apply discounts or if there is similar modules. I think an easy way to tackle this is to set two price tags to each merchandies; one for standard price, another for discounted group's price. But by doing so, it will limit the discount to only one usergroup alone. Ideally, I would like to have multiple user levels with different discount values for each product. That definetly will need some work, and thus is the biggest set back I can foresee right now.
I think, given time and assume that I can work out those above three things, integration shouldn't be possible. If all else fails, another way to do it is to use snipplets of OSC's code, and build it from vB. However, that way it wouldn't be an integration, but quite a large hack...
I would definetly like to hear some opinions on this one though. Seems to be quite a bit of work ahead of me and I'm not even quite sure where to start yet