I tend to think PNS or VPS are a tad small for WN. Now I know that WN does serve other markets similar in size to PNS, but in more recent times WN has been trending toward larger markets. One key to success for WN is constant growth...which keeps costs in line by always adding new employees to balance out the more senior ones.
A market like PNS might be profitable for WN, but how much would it grow over the course of 1, 3, 5 and 10 years? Many of WN's smaller markets have really stagnated...when was the last time WN added a flight at MAF, JAN or GEG?? That's one reason why WN chose PHL...lots of growth potential.
There have been rumors for years that VPS and PFN would build a joint airport in between the two cities and that WN would start service. However, I don't see that happening anytime soon. Until VPS gets its new terminal built, I can guarantee you that you won't see WN. VPS also has limits on the number of commercial flights allowed because of the fact that its also an Air Force base. WN might bump into that ceiling.
Should note that PNS does already have one low-fare carrier, Airtran, which seems to be doing ok. I don't know if PNS could support two of them.
Could PNS or VPS happen? Yes, but I don't see it in the short term.