Here are a few simple plugins that you may add to vim that'll help you write and test Ruby code. NOTE: I used pathogen to install these plugins.

vim-sensible - several decent additions to standard vim that make development easier, i.e. having backspace in insert mode, immediately go to next search result, etc. It also loads matchit.vim - which allows you to use the % key to jump from start/end of Ruby classes/methods/blocks.

auto-pairs - Automatically closes parenthesis, quotes, etc and puts your cursor in the correct place to begin typing. It also handles deletion of brackets properly.

vim-surround - Allows you to wrap existing text in parenthesis/quotes, and provides ability to remove brackets and quotes from existing text.

vim-endwise - Automatically adds "end" when writing Ruby code, for e.g. when defining a module/class/method/block, etc.

vim-rspec - Allows you to run rspec tests in the current terminal. I prefer this compared to using a separate terminal window for rspec output as I prefer having the whole screen dedicated to test output during failures.

vim-ruby-refactoring - This has a number of functions that allow you to refactor things like variables, extract methods and so on.