Preface: I have a mid-size forum running VBulletin (.5million posts, 500-1000 users at peak times, growing quickly) that is becoming a real pain to handle. I'm currently on unmanaged hosting which was financially necessary when it was first setup.
I don't have the time to manage the server anymore, and I would like to move to a company that is able to both manage the server entirely, and also assisst intelligently with the scripts (vbulletin in particular) - someone that can manage the server with an eye to what is required to keep a fast-growing php/sql site operating well, as well as diagnose and correct issues when they arise - and if it means going into a script now and then or advising on precise corrections, that would be a plus.
Location is north america, my budget is flexible (400/mth is an upper limit, but not hard), my coding knowledge is moderate: I know what needs doing, but I don't have the knowledge or the time to invest in getting the knowledge to do it myself.
I hope that's not too vague. I see a few others in this forum with a similar problem, but the 2 threads with the best advice had very specific problems that don't apply to me.
I would probably go for the sysadmin company as if you pick up a good one, (do some searches but PSM comes out quite well) it means they will become familiar with your site and should you ever need to move to a bigger server they can help migrate. Also it means that if you don't like the management you can swap without having to change datacenter