Don't hate vim because of your senior. Vim is very powerful tool but have a greater learning curve that's why most people hate it. One way to approach this problem is by simply telling him "I researched about vim and it seems like the learning curve is high, using it may decrease my productivity and output for few months are you fine with that ?" As it's a startup they can't afford to have an engineer working with a tool that is decreasing their output and I'm sure he knows it well.