It's a good business idea to bring in more money, but this would affect regular users. For instance, What Is Life was one of your more popular beats Slantize, and seeing as it's off the site now I can only assume someone purchased it. With your idea, only the VIP members would ever have heard of that beat and no one else, so it starts to become that the instrumentals the regular users are allowed to download/listen to are just the leftover scraps of what VIP members didn't like. Looking at it like this, if someone new to the site is debating whether or not to become a VIP member, your best work wouldn't be available to help them make a decision because it would have already been bought by someone else.
Another issue with your idea is what if some VIP members simply upload your beats to ZShare or some other file hosting site? What would motivate people to buy the cow if they can get the milk for free?
Unfortunately with your current plan you're not going to be able to avoid the first problem, and the second will probably happen though to what degree I'm not sure of. Things to think about. Perhaps a 30 second mp3 of exclusive VIP beats would help? Although one of the reasons I keep coming to Shadowville is because I don't even bother with 30 sec. beat mp3 sites since I can't really afford to spend 20-40 bucks for every beat I like, but I suppose if you limit it to only a few beats it'd be bareable and maybe even incline people to purchase VIP subscriptions since you're just giiving them a taste of your best work causing them to want more.
I would buy a VIP subscription btw, even though I've never bought anything on this site before