I agree that 30 days is a lot of pressure -- you may want to push your test date back to 60-90 days to really give it a shot. There are no magic ways to improve except significant study time and good strategies.
For basic Vocab improvement:
Start with your grammar skills first. You can ignore most of the challenging vocabulary on sentence corrections as long as you identify what part of speech each word is, and how it functions within the sentence. To do this, you'll need to spend some time with a solid English grammar review book. I recommend pairing a heavy-duty review book, like the Oxford Guide or those published by McGraw-Hill or Longman, with a "fun" book like Writer's Express or English Grammar for Dummies. I would also get MGMAT SC if you haven't already worked through it.
Read and listen to high-quality English publications. My recommendations include The New Yorker, the Atlantic Monthly, the New York Times, the Wall Street Journal, or any scholarly journal that you find interesting. Listen to NPR or audio books of English-language classics. Set a regular schedule for your reading and stick to it. Even thirty minutes a day will help you conquer Reading Comprehension.
Make a vocab journal. Write down any words you don't know as you encounter them. You'll start to notice that certain words appear over and over again. Make flashcards for the ones that have tricky definitions or mean the opposite of what you'd expect.
Think like a thesaurus. It is much easier to memorize synonyms for words than their full definitions. Start grouping words together mentally (and on paper) according to their meaning. For example, words like "pusillanimous," "poltroonish," and "timorous" would go on the "shy" list.
Seek out Advanced CR for harder vocab words. Aiming for a 700+ score? Once you've practiced identifying the conclusion, evidence, and assumptions and are confident with the Critical Reasoning question types on the GMAT, consider buying an LSAT practice guide like the LSAT LR Bible. The LSAT has significantly more challenging CR questions and the format is the same as those found on the GMAT. Don't neglect your GMAT practice, but if you can master the LSAT CR, then the GMAT questions will start to feel easier.
In general, I would spend time with developing a good knowledge of CR from Powerscore CR (as others mentioned) and work out your own RC strategy. You can find lots of blogs here on BTG -- every tutor has his/her own method. If you need more help, take a look at Veritas RC, or consider doing 3-4 hours with a good GMAT tutor to solidify your RC strategy. Good luck!
