Vim highlight lines of Ruby that are not covered with tests. When the test suite runs, SimpleCov will generate a "coverage/raw" file. Next, the Vim function can be used to read data from that file for the current buffer and mark lines not covered with tests.

test_helper.rb

require 'simplecov'
require 'coveralls'

# writes out a "coverage/raw" file to be read by Vim
class RawMissingLinesFormatter
  def format(result)
    File.open('coverage/raw', 'w') { |raw|
      result.source_files.each do |file|
        file.lines.each do |line|
          if line.missed?
            raw.puts "- #{line.number} #{file.filename}"
          elsif line.covered?
            # uncomment to also have covered lines displayed:
            # raw.puts "+ #{line.number} #{file.filename}"
          end
        end
      end
    }
  end
end

SimpleCov.formatter = SimpleCov::Formatter::MultiFormatter[
  SimpleCov::Formatter::HTMLFormatter,
  RawMissingLinesFormatter
]

SimpleCov.start do
  add_filter '/bundle/'
end

vimrc

" This is a local, per-project `.vimrc`.
" It maps `H` to read in line coverage from the "coverage/raw" file and
" mark uncovered lines as errors using Vim :signs functionality.

highlight SignColumn NONE
sign define covpos text=• texthl=String
sign define covneg linehl=Error text=𐄂 texthl=HelpDebug

function! s:SignCoverage(file)
  let output = system('grep "'.a:file.'" coverage/raw')
  let lines = split(output, '\n')
  exec 'sign unplace * buffer='.bufnr('%')
  for line in lines
    let [ type, lnum, fname ] = split(line, ' ')
    let name = type == '+' ? 'covpos' : 'covneg'
    exec 'sign place '.lnum.' line='.lnum.' name='.name.' buffer='.bufnr('%')
  endfor
endfunction

map H :call <SID>SignCoverage(expand('%:p'))<cr>