Vizio isn't a manufacturer—they buy panels from other producers. Co-founder Laynie Newsome cites the fact that in the showroom, LCDs are brighter and lure more customers, but there are presumably other factors, including the fact that Panasonic, the largest plasma manufacturer, has had to cut back on 2009 production.
The Panasonic cutbacks allegedly mean having fewer panels to sell to partners like Pioneer. Since Vizio did buy source panels from Panasonic, maybe Vizio got screwed out of panels too. The good news is, Vizio is working hard to gain LCD market share by putting out sets with features like LED backlighting—hopefully at low low prices! What this means for the future of plasma is still unclear, but it can't be good.
Note: Pioneer has not officially confirmed that it is out of the plasma business, but in two statements on the subject, they have not come close to denying it either. [NYT Gadgetwise]