I am having a similar issue, where I have a daily delta qvd load into a QVDs folder running. From this folder I would like to read only the QVDs, which haven't been read already in my report application. There's a datestamp on the delta QVDfile but it could be that there are several QVD loads per day. I know the delta load logic, but don't know the syntax!
Firstly create a Loaded.qvd with all the loaded delta. Then in report application compare this Loaded.qvd with the delta QVD files in the QVDs folder, and load only the ones which haven't been loaded already. Then add these into Loaded.qvd. Just like dathu.qv proposes.