How can I excel in the vocab and grammar section of the SAT?

You can start reading now- it's not too late. Even reading the news from a decent paper like the New York Times will expose you to new words. You can also study prefixes and suffixes- these can clue you in to what a word you don't know might mean. Another exercise that might help is opening a random section of the dictionary and copying 10 or 20 definitions a day. Probably the most helpful things you can do is just read a lot- if you're not taking the SAT til October you'd be surprised how much reading you can get done.