from.rb

get '/:id' do
 @vent = Vent.get(params[:id])
 erb :show
end

# show post
get '/:id.xml' do
 @vent = Vent.get(params[:id])
 builder :show
end

to.rb

## Using ? will make the previous section optional
## NOTE the ? after the .

get '/:id.?:format?' do
  @vent = Vent.get(params[:id])
  case params[:format]
  when 'xml'
    builder :show
  else
    erb :show
  end
end