Bitcoind server strategy. How much disk space?
Hosting Bitcoind on a VPS is a very bad idea. I cannot stress this enough. The VPS provider will not have adequate security standards. You increase your risk profile dramatically as the combination of software running on the host server can cause a problem for you and your users. Please. Don't.

That said, if you do want to, it all depends on how transactions will be dealt with in the future. As a VPS is usually fairly flexible I'd take about twice the blockchain now, and just upgrade when you're above 80% disk space usage. That gives you some wiggle room.

You may consider using a light client, perhaps with increased connection count (more than 8). That will totally remove your disk space hunger and it's reasonably secure.