When you install vBulletin, in the admin section, there is
a way to transfer your old forum to vBulletin, without bridging
any tables. It is all automated. You select your old board,
select the database, and done.
You can probably find scripts on Google for transfering
one forum to another, for most forums. They definitely come
standard with vBulletin and Invision Board.
I would find a way to buy vBulletin - it's worth the money.
I think it is the other way around - you cannot afford to
NOT have it. The costs of having another cheap forum are
to great - losing all your topics, posts, visitors, etc. Just
put up a paypal donate button on your forum and ask
users to contribute so you can buy it. My forum has $240US
sitting in PayPal, and donations pay for our monthly hosting
as well.