Hello, with the setup you have I don't see anything that will cause an issue. If you have any more issues after moving up to the higher vps with more memory then it will most likely be caused by the host and/or other vps users.
I have a vps setup for vBulleting forums only and have about 3-4 times your usage and I am only using about 400mb max.... and that's running fastcgi with children which uses much more memory. I can scale it back to about 200 mb without fastcgi and it runs great as well.
What you will run into as you grow is needing a proper mysql setup though. This can make a big difference on how well a site functions in a vb setting.
As for specific processes to monitor there is really nothing that vb uses other then mysql and php for it's operation.
you should be fine.