Cleaning pac n play

So I read that using a lil baby detergent with baking soda and vinegar is good for cleaning a pac and play. To soak in really hot water, rinse and dry. Well I did that and now the mattress looks stained and dingy. I'm so upset. Mind you I've only used the pack and play for 3 weeks as a bassinet. My son only peed and spit up a lil on the mattress. I'm so confused as to why it now looks so dirty. Any advice on how to remove the stains and how to properly clean the mattress in the future. Thanks in advance