Yeah, use secure coding methods when rewriting VBulletin.
I wouldn't blame load problems on SQL injection. The symptoms of SQL injection would be data being modified or accessed without intended permission.
You probably only need a sysadmin knowledgeable in optimizing the database server.
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Load problems in forums that aren't from the pure weight of traffic typically come from the extensions/add-ons.
The programmers of these will have made new queries against the database that the database designers weren't anticipating, and the extension programmers rarely think about whether they should also be adding indexes to the database, or modifying their queries to make better use of the indexes already there.
Setup the slow query log as explained in the mysql manual to see if you have any queries that take a long time to run, then search for them in you forums code. If they are in an extension, consider removing it, or if your SQL is up to it, analysing them to determine how to rewrite the query or add indexes to make it faster.