is 1tb bandwidth and 100gb space going to cut it say 5 years from now if I have over 500,000 members, over 9000,000 threads on the forums and 500 members online at once.
Hard to say -- it might cut it just fine, or you may quite possibly need an upgrade -- but they also offer larger packages than that, so (assuming you're still willing to pay for it) there's no reason you couldn't simply upgrade the plan -- and you'll actually get what you pay for, unlike many supposedly "unlimited" plans where you're suddenly hit by extra charges or have your site throttled when you start using more resources. You might find out more from the operators of larger vBulletin based sites on what sort of resources it uses under different traffic loads if you really want to be more sure about potential future growth.
Honestly, you'll be doing really well if your site actually gets that busy, so I wouldn't let it be a huge concern right now -- you can always deal with it if and when it happens, as long as you choose a host that a) offers viable upgrade options, and/or b) isn't a pain-in-the-ass if you later decide to migrate to a different host, which is unfortunately a known issue with GoDaddy.
I don't know if Media Temple are the highest specs you can get, but their infrastructure and support are both top-notch. You'll have to shop around more yourself if you want to look for alternative options -- let us know if you find any!