It isn't clear if this is true.
"in fact they reported it"
VAN has not stated the issue was reported by the Sanders campaign. The claims that the Sanders campaign had reported an earlier issue are refuted in the OP, which states they had reported issues with another vendor's software.
It is possible the bug was abused:
"The database logs created by NGP VAN show that four accounts associated with the Sanders team took advantage of the Wednesday morning breach. Staffers conducted searches that would be especially advantageous to the campaign, including lists of its likeliest supporters in 10 early voting states, including Iowa and New Hampshire. Campaigns rent access to a master file of DNC voter information from the party, and update the files with their own data culled from field work and other investments. After one Sanders account gained access to the Clinton data, the audits show, that user began sharing permissions with other Sanders users. The staffers who secured access to the Clinton data included Uretsky and his deputy, Russell Drapkin. The two other usernames that viewed Clinton information were “talani" and "csmith_bernie," created by Uretsky's account after the breach began. The logs show that the Vermont senator’s team created at least 24 lists during the 40-minute breach, which started at 10:40 a.m., and saved those lists to their personal folders. The Sanders searches included New Hampshire lists related to likely voters, "HFA Turnout 60-100" and "HFA Support 50-100," that were conducted and saved by Uretsky. Drapkin's account searched for and saved lists including less likely Clinton voters, "HFA Support <30" in Iowa, and "HFA Turnout 30-70"' in New Hampshire. Despite audit logs, Weaver said at the news conference that NGP VAN has told the campaign that no Clinton data was printed or downloaded."
http://www.bloomberg.com/politics/articles/2015-12-18/sander...
We are social creatures; we have a deep need to align ourselves with groups of others. And the evidence points to our having a deep need to argue as well.
The Sanders campaign had in October reported an unrelated software issue in a non-VAN system.
They did not report THIS issue to the DNC or NGP VAN, but claim they were gathering information about the breach for the purposes of reporting. Based on my reading of the OP, the breach was discovered by NGP VAN employees.
That is pure conjecture and I'm not even sure the Sanders campaign would agree with you considering they have fired a staffer over this.