Vernon's debut at an international athletics competition was at the 2004 IAAF World Cross Country Championships where he competed in the 8,000-metre junior men's race; finishing 64th in a time of 27minutes and 12seconds. Vernon's next international competition was the 2005 IAAF World Cross Country Championships where he again competed in the junior men's race, this time finishing 48th. At the 2005 European Athletics Junior Championships he competed in the 5000metres; finishing 4th in a time of 14minutes and 27seconds. In December 2005 he won his first international medal in the European Junior Cross Country Championships finishing 2nd behind Hungary's Barnabás Bene. He then competed at the 2007 European Cross Country Championships and finished 3rd in the men's under-23 race. At the 2007 European Athletics U23 Championships Vernon competed in the 5000metres and finished seventh in a time of 13minutes and 59.24seconds. Vernon competed in the 2008 World University Cross Country Championships, finishing sixth. Also in 2008 was the 2008 IAAF World Cross Country Championships where Vernon finished 115th in the 12kilometres senior men's race. His next major competition was the 2008 European Cross Country Championships where he competed in the men's under-23 race. He finished second behind Italian Andrea Lalli. Vernon competed in his fourth World Cross Country Championships in the 2009 edition. He competed in the senior men's race and finished 78th. At the 2009 European Cross Country Championships in Dublin Vernon competed in the men's senior race; finishing 12th.