Its not that Vardy would contribute more than Kersten its that he would free up Kersten to play the third tall roll and exploit the third defender. If Brown or Walker were good enough to contribute then we could use them as the second tall to free up Kersten but they aren't anywhere near the standard. Its questionable whether Vardy would be, and that's why getting rid of Pods was a dumb mistake even if Vardy was fit, but Vardy was well ahead of Brown and Walker in being to hold off defenders in marking contests and compete for the ball in the air and that is exactly what we need.
Walker averages more disposals (8.2 to 7.8), the same marks (2.6), way more tackles (2.8 to 1.9), and a whopping 0.1 goals per game less (0.8 to 0.9) than Vardy does. So, apart from the ubiquitous "well the match committee prefer him - therefore I'm right" argument, how exactly is Vardy so much superior to Walker? Hype undoubtedly. On actual output, there's very little difference.