Like the other people have said, you might as well just get a free forum like phpBB, besides, would you really want to waste nearly 200 dollars to get a vB forum, and then risk wasting money when you find out that the activity decreases? Because that could be a possibility.
Just get a free forum, work very hard at getting more members with originality, a wicked skin, and many other goodies, and if it actually becomes a successful forum, THEN use your money to buy a vB.
Anyways, I have not heard of vBulletin Lite, have you checked on Google? If it exists, Google will find it for you. =P