The #1 Hands down reason Viking captured and is taking names on its boats is due to the continual R&D and reinvestment back into the plant. My opinion is that Hatteras gave up on their bread and butter (SF's) in the late 90's. THey didn't have any new tweaks to their product. During the same time, Viking came out with prop pockets in the hull, vacuum infused layup's, Viper independent rudder steering, and incorporated the Penta running gear platform (albeit not wide spread yet due to HP). During the same time, Hatteras changed its interior design 3 times, then tried to relaunched a stripped down SF called the GT. Yes Hatteras changed some window lines, but nothing performance oriented. That attitude continues from the mid to late 90's through today (justifiable today when your selling the platform). That is why I think Viking took over the market and Hatteras did not.
The thing to watch will be Spencer... He makes as many customs as the rest of the custom builders put together. Paul and Daniel are putting cutting edge technology and performance enhancements into their boats faster than Viking can (simply due to being non-production). My opinion is that "any" custom will have about a 8-10 year jump on any "production" boat, simply due to the mfg process of a production boat (it has to be affordable). in fact, a Spencer boat will be less expensive (not equal price) than a Viking boat and have way more technology into it (especially in the 40 - 60 foot range). Once the days of Production building die (now, unless you want a Carver or Bayliner), I think you will see Spencer and a couple of the other larger custom yards start to take over the market.