The documentation states that the third parameter
directoryPathInArchive (String)
Specifies a directory path to use to override any path in the file name. This path may, or may not, correspond to a real directory in the current filesystem. If the files within the zip are later extracted, this is the path used for the extracted file. Passing null (Nothing in VB) will use the path on each of the fileNames, if any. Passing the empty string ("") will insert the item at the root path within the archive.
So if you always want to have the files added to the root of your zip archive, change
loanZip.AddFiles(documentPaths, false, zipDestinationPath);
to
loanZip.AddFiles(documentPaths, false, "");