I really don't think flash cards will help that much, unless it is strictly vocabulary that accounts for your lower verbal score. But it sounds more like the problem is reading comprehension, which is in part based on vocabulary but also your ability to identify the main idea of a paragraph. Also, vocabulary won't really help with sentence correction section, which is all about grammar. Rather than working with flash cards, I would read the paper and magazines daily and work on creating 1 sentence summaries of each paragraph (even if only in your mind).