I run a linode VPS with 512MB of memory and I was wondering if a PHP accelerator or PHp cache is a must have?
The point of the server is a vBulletin forum.
Which one would you recommend if so?
protip: 99% of systems administration is trial & error
no one can tell you what the memory-footprint/performance tradeoff for your version of vbulletin running your inevitable customizations at your traffic levels, sharing a tiny vps with whatever else you're doing, is going to be.
that said, apc is pretty much standard-issue at this point. I'd have to have a reason not to set it up on any new php environment, and you haven't shown one yet.
edit: and if this is apache/mod_php you'd better have your maxclients/serverlimit down at like 10
The best way to try this is doing some tests and testing the performance.
I used Accelerators sometime ago when I deployed SquirrelMail to 100k users, the performance increased 5 times. After that I got other problems and solved using a reverse proxy in the front and distributing the load through more machines.
But it's a great solution and the unique thing you can do is test.