The Kobo Vox and many other Google Android Tablet comes with 8GB of Space, but many a little bit of space is used for the Google Android operating system and unremovable Apps, so you can't use all of its space.
If you want to save mpre space, I recommend only installing Apps you would use, and uninstall Apps which you previously downloaded from the internet, but never or hardly use. Plus, moving Apps to SD, and personal files to a MicroSD memory card. The Kobo Vox supports a 32 GB Micro SD Memory Card in size, so you can have plenty of HD video files, high bitrate MP3 sog files, and High resolution files on it to view on your Vox while freeing up space by moving personal files like pictures, videos, and music files from the internal SD to your MicroSD card which can be up to 32 GB in Size.
Moving Apps to SD will make Google Android Gingerbread on the Kobo Vox start up a lot faster if your Internal Memory is almost full, and reduce the chance of not in enough memory error when installing and downloading apps.
You can also go into your web browsers settings to delete your history, temporary cache files, and cookies which could save a few MBs of space if you browse the web often on your Vox.
In Google Android Gingerbread, you can go into your Application settings, and click on Manage Apps, and click on the All Section on the top, and uninstall App or clearing Data, Clearing Cache, etc.
Uninstalling Apps are the best way to free up free space. You can always re-install the app by running the APK installer file for your App with Android if you have the App APK or re-downloading the App APK file from GetJar, Amazon App Store, Google Android Marketplace, etc.
If you don't want to uninstall the Apps, you can click Clear Data which will just delete your save files, but not the App, so when you need to use it, you need to agree to the Terms of service, change the settings again if you have customize settings, and restart games because your Save file was deleted when you cleared Data. You also might lose notes in the App like Annotations for eBooks, and bookmarks when you cleared the data.
You can also move Apps to SD or Apps to Internal Memory in the App info section to free up space on your SD or Internal Memory by clicking the "Move to SD" or "Move to Tablet/eReader" button depending on where your App is installed on. I recommend moving Apps to SD since the internal memory is usually more full, and if your Internal Memory is very full, it can make your eReader tablet start up slower, and possibly crash more with certain apps like "Cut the Rope" which seems to crash more often when the Internal Memory is very full.
Clearing the Cache by clicking "clear cache button" will just delete your temporary cache file. The Cache file is used to speed up your start up time for apps. If you clear the cache, your next start up time will be longer until the files are cached again. It is safe to clear the cache for all your apps usually since they will be just be remade the next time you start the App.
Other Tasks to free up disk space on your Google Android Tablet.
Delete or "Cut and Paste APK files to your Computer by using a USB cable for your Tablet to connect to your PC". After installation, you do not need the APK files since your Apps are installed, so you don't need your APK files for your Apps to run.
You can also go to GetJar, Android MarketPlace, App maker Website, etc to see if there is an updated version of your App to update to the latest versions. Newer versions of the App may use less space, are more stable, and are more easy to use.
One of the Best ways to save space on your Tablet, is moving/cut and paste all your files you downloaded from the internet like APK, MP3, pictures, videos, etc from your Tablet to a MicroSD Memory Card via a File Manager like File Expert. This means your Tablet will have more space on its SD memory. Using a MicroSD memory card also makes copying files from your tablet to other tablets, PCs, Cell phones, eReaders, etc a lot easier since you just need a USB MicroSD card reader or Built-in MicroSD card reader to insert and read the files from the card.
Also, you have more space to install Apps to the internal SD, eBooks, and App data which needs to be on the internal SD or Internal memory, and can't be saved to MicroSD card easily.
You can also delete eBooks you already read, or don't like to read by holding down your finger until a menu appears and tap delete on the menu to delete the eBooks from your Tablet.
Listening to lower Bitrate MP3, watching lower quality video files which are not HD, and viewing low resolution photos on your Tablet will also save a lot of space since lower quality files use less space. You may need to use a video converter or sound converter if you need to convert your files to a different format which works with Google Android. MP3 music, MP4 video, and JPEG picture files can be opened with Google Android OS.
Using online services websites like YouTube, Google Docs, Gmail, Google Calender, Onlive, Gaikai, NetFlix, Hulu, Crackle, Spotify, Rdio, etc would save you the most space since online websites don't use any of your space because no big work files, movies, songs, games, etc are saved to your Tablet. Everything is being run off a web browser or client app on your Tablet, and the data files are on a powerful computer server in the internet. All you need is a web browser, and wireless internet connection to work on your work, and enjoy yourself when watching videos, listening to music, gaming, movies, and socializing when online.