Probably one of the best questions to ask your potential VPS Hosting Provider, is why you need one. Find out why they think you need one and what size you need. If you can provide any logs or stats from your existing site, they should be able to help guide you on what VPS level you might need.
Be aware of places that steer you to the most expensive or dedicated servers and then try to pass it off as being an option that allows you to grow into it. Any provider worth their salt should be able to get you on a hosting package that meets your current needs with options to upgrade as needed. Also, just because it's more expensive, it doesn't mean it's worth it.
Get manged hosting when possible - this lets you do what you're good at, and the hosting company do what they're good at. Talk to Artashes here on the forum and ask him how much he likes Manged Hosting solutions
Find out about backup solutions, monitoring, firewall, conflict resolutions, uptime guarantees, OS re-installations, control panels, virus scanning etc etc. I'd recommend going with a company where they do everything, and you just run your forum - this way you know you're always up to date with the latest patches and software (aside from your forum software).