Best app for advanced vocab building

Im looking to take the GREs again to apply to get my 2nd masters, and I only wish this app was available 17 years ago when i first took them. Rather than drill me on random big words, many of which I already know, this app enables me to generate a list of only the words *I* want to learn, and then quizzes me on them in an ingenious & most difficult way that really reinforces long-term memory, which is by skipping the multiple choice, displaying a random definition, and then forcing me to recall what word Ive saved best fits it. Correct spelling counts, and the app wont drop a word off your list until you correctly recall it in random order 3 times in a row. Difficult to describe, but its hard & effective. My only beef which gives it 4 stars is the difficulty syncing Word Vault among different iOS devices w/o Dropbox account. It claims this is possible via iCloud back up, but theres no instructions on HOW!!!

TallisonR about Word Vault: Vocabulary Builder, v2.0.2