After returning from a mission project in England, he was asked by the new General Conference President R. R. Figuhr to continue with the Christian television program. Six years earlier then-president J. L. McElhany had convinced Vandeman to try television as a means of reaching others with the Gospel. As such, he created a six-month experimental evangelistic effort for television. At the time, he wasn’t able to get the financial support he needed, and temporarily put the effort on hold.

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In the mid-1950s Vandeman started work on a series of television programs called "It Is Written", which he planned to air for several weeks in an area as a warmup to an evangelistic program.[3] In spring 1956, It Is Written launched its first telecast in black and white—a full-message, Bible study telecast in Washington, D.C. The program later became one of the first religious television programs to air in color.[5] The program's title was based on the Bible verse Matthew 4:4, "It is written, man shall not live by bread alone, but by every word that proceeds out of the mouth of God."

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Vandeman ran the first It Is Written campaign in Fresno, California in 1958, and later in Washington, D.C., Detroit, Philadelphia and other cities.  The telecast was launched to all of California in 1962, and this effort was followed by a month-long series at the Los Angeles Memorial Sports Arena. By the mid-1960s it was being broadcast internationally on a weekly basis.

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In 1971, the It Is Written production studios moved to the Adventist Media Center in Thousand Oaks, California. In 1975, Vandeman began conducting Revelation Seminars. The seminars consisted of a one-day, eight-hour Bible study followed by a luncheon. Over a course of 10 years, tens of thousands of It Is Written viewers traveled hundreds of miles to attend one of 300 seminars.

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In 1979, the ministry’s success was noted by Excellence In Media, a non-profit organization dedicated to promoting excellent family-oriented programs by honoring media with Angel Awards. Vandeman and the It Is Written team were given their first Angel award. They would ultimately received a total of 10 Angel Awards from Religion in Media.  Vandeman was later presented with an International Distinguished Achievement Gold Angel award — Excellence in Media’s highest honor. In 1980 he received the Religious Heritage of America Faith and Freedom Award for Television Religious Personality of the Year. He was a member of Religious Heritage of America, the Strategy for the Elevation of People Foundation, and the Year of the Bible Committee.  Vandeman was invited to presidential briefings during both the Reagan and Bush administrations.

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By the 1980s, It Is Written had more than 600,000 regular viewers. That number surpassed 1.5 million in the 90s. In 1990, the It Is Written production team and Vandeman traveled to the Soviet Union to tape "Empires in Collision," an eight-part series. It Is Written was one of the first religious telecasts to be aired on Soviet television. In 2000 it was broadcast in 8 languages to over 150 countries.[8] It was a "pioneering force" in Adventist evangelism.

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One of his most popular series was "What I like about...", which investigated shared beliefs between Adventists and Baptists, Methodists, Catholics, charismatics and others. See: Seventh-day Adventist interfaith relations.

Vandeman founded the New Gallery Centre in London.

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He served as the primary speaker of It Is Written until his retirement in 1991, when Mark Finley succeeded him. He died on November 3, 2000 at age 84, at his home in Newbury Park, California of heart failure while asleep.  He was survived by his wife, Nellie of Camarillo, California, and by children George Jr., Bob, Ron, and Connie Vandeman Jeffery.