input: Write a python function to perform sentiment analysis using the VADER sentiment analysis tool.
  assert sentiment_analysis("This product is excellent") == "positive"
  assert sentiment_analysis("I am very unhappy with the service") == "negative"
  assert sentiment_analysis("The movie was okay, not great but not terrible") == "neutral"
output: def sentiment_analysis(text):
      from vaderSentiment.vaderSentiment import SentimentIntensityAnalyzer
      
      analyzer = SentimentIntensityAnalyzer()
      sentiment_dict = analyzer.polarity_scores(text)
      
      if sentiment_dict['compound'] >= 0.05:
          return "positive"
      elif sentiment_dict['compound'] <= -0.05:
          return "negative"
      else:
          return "neutral"