Will you monitize this site throughout, what have you estimated in your business plan for the amount of money you will make. It makes a difference if you are generating income or not.
Shared hosts try to maximize the number of sites on a single server; therefore if one of your goals is to create a good user experience (professionally) then shared hosting is not good for forum software, a shared host will become overwhelmed very quickly using vbulletin. If you can find a reasonably priced VPS then start there.
If you are in a market where you can pay for what bandwidth/storage is used then you may consider Cloud hosting like Amazon S3 or EC2, however for most non-static sites that generate a lot of traffic but little income, cloud hosting can get very costly.
You will work with your host regarding your resource consumption and will consult with them when you know that you are hitting your VPS resources too hard.
It is easiest if you choose a host that has a good path/pricing from shared -> VPS -> dedicated, so choose your hosting carefully. If you haven't reviewed the free "Find a Great Hosts in 5 Steps' document that SitePoint has, I'd recommend you look through this.