You went to the RCB and ? Found records, no records there?
Did whoever lead you to believe the Cornelius family were buried in the family vault say where that vault was?
When you're led to believe some information about an ancestor it's always helpful to query the source about where their information originated. That can save time and effort searching locations that may not be relevant to your family research.
Did you get a copy of Henry's death cert? If not you can order it from the Ireland General Register Office http://www.groireland.ie
and click on "apply for cert".
Best wishes in your research.