With the assumption that your vertical is large enough to warrant it's own vertical-specific ad network, building a good ad network is fundamentally about your salesmanship to both the publishers and advertisers.
As with any "network" or "exchange" – you have the classic chicken and egg problem between people who will advertise on your network (advertisers) and people who will host your advertisers' ads (publishers).
Start with looking on which side of the equation makes more sense in your vertical. In some vertical, one side may have more leverage then the other or one may be easier to build.
On top of that, you'll need some kind of platform or technology solution to manage your sets of publishers and advertisers and to manage the traffic, costs, and inventory.
There are many providers who let you create an "ad network" as a software solution, if your vertical is niche then this is your best approach. Attempting to build the software from scratch is a monumental task, so unless your vertical can not be served with any of the following solutions, it's probably not a good approach since you'll probably need a lot of capital to do this.
This list is not comprehensive, and I'll add to it (or please suggest edits).
- from Adzerk seems like a good fit.
- (Formerly Adify) BYON (build your own network) http://www.coxdigitalsolu
- BuySellAds Private-Label Solution: http://buysellads.com/pri
- InfluAds: http://influads.com/netwo
- LinkTrust.com (Offers a "Network" solution)
- HItPath.com (May be used to manage your "ad network")