Where are OS X document versions stored? Are they secure?

Versions are stored on the same physical / logical volume as the original. This Q&A has all the technical details, but in short, look in the root user owned folder /.DocumentRevisions-V100 on each volume. If you create a disk image, the user that mounts the disk image normally owns the /Volumes/whatever/.DocumentRevisions-V100 directory where changes to documents on that volume are stored.

• Do OS X Lion's Versions and Resume features store the cached data for encrypted .sparsebundle images in a secure way?

So - if that volume is FileVault encrypted (or a secure disk image) then the version is as secure as the original.

When you copy a file by attaching it, you only send the current saved copy - not any previous versions - so then again - it's only as secure (or not) as the medium into which you send that file.

So - versions doesn't do any additional encryption and it's really up to each application to decide whether it enables or disables versions and whether the data it writes is also obfuscated / encrypted - but the presence of versions neither adds nor subtracts security to a system.