Vanna White is feeling like a third wheel now that Ryan Seacrest has taken over for longtime host Pat Sajak on Wheel Of Fortune, according to sources. Insiders say she is considering following Sajak out the door even though she is fond of Seacrest — and the show’s producers are in a tailspin fearing a major fan revolt, RadarOnline.com is told. After her TV partner and close friend Sajak announced he was leaving, the veteran letter wrangler admitted, “I thought maybe I should retire, too.” But 42 years of living in his shadow prompted the 67-year-old beauty to be enticed by “the thought that ma…