Jerome was a valid who had genes that could be considered as being optimum. There were no defects and as such could get almost any job that he wanted. He was a former swimming star, but when he came second in a high profile competition, he thought that he had failed at life and attempted to commit suicide by jumping in front of a car. He didn’t die but was now paralyzed from the waist down. As this happened out in the country somewhere, no one knew about his new disability, and Jerome was happy to keep his failure secret. He distanced himself from society and lived as a hermit where he could quietly be disappointed with himself.
So as Jerome had ‘perfect’ genes and had given up on his own life, he was the perfect person for Vincent to ‘become’. He didn’t want to use his life and as such was happy for Vincent to able to use it. Even though Jerome now had a purpose for his life in helping Vincent become him, he was still depressed about his failure and eventually did kill himself at the end of the movie.
Jerome was similar to Vincent in his basic appearance. He was of a similar age and also had dark hair. While they looked similar, Jerome was a fair bit taller, and they both looked very different from each other. But as Jerome had been hiding from society, no one had seen him in a while and Vincent could be made to look enough like him to pass.