You need to use the information in the VWAttachment object to fetch the correct document from the Content Engine.
for example:
com.filenet.api.core.Domain domain = (...you need to get this);
VWAttachment vwAttachment = (...you already have this);
ObjectStore objectStore = Factory.ObjectStore.getInstance(domain, vwAttachment.getLibraryName());
VersionSeries vs = Factory.VersionSeries.fetchInstance(objectStore, new Id( vwAttachment.getId()), null);
Document document = (Document) vs.get_CurrentVersion();
ContentTransfer contentTransfer = (ContentTransfer) document.get_ContentElements().get(0);
InputStream inputStream = contentTransfer.accessContentStream();