Mihawk used only so much force to pressurize rookie Luffy, while giving him the chance to escape. Obviously, for Vista Mihawk would need far more strength than he allowed himself to use against Luffy, and as Vista interfered, Luffy passed the test (having the ability to make others his ally). It wouldn't make sense for Mihawk to use as much strength as needed to overwhelm Vista, because that amount of power is not something rookie Luffy could handle.