[janslu]

Some of it is really important stuff. I think rounded corners may slow down page rendering on very slow, outdated computers. The real problem with vbulletin is the amount of requests generated by styling and inefficient coding. All those icons, gradients, separate javascripts and css files are being loaded from one server. Take a look at yslow and see the numbers...
I have no idea why it wasn't addressed properly by the dev team (single css, single javascript! try using css sprites for the images). It was released in 2009 for ... sake! We all know the impact of number of requests on page rendering time...
So... Use CDN for static files delivery where possible. Try spreading out between a couple of subdomains. You can have one amazon s3 bucket with multiple cloudfront subdomains (ie. static1. static2. etc). In StyleVars there is a section where you define image paths - put the whole link to CF there (http including). When uploading to s3 - remember to set your expiration dates properly. This will help with consecutive page loads. Setting the header is not easy, but it is possible.
As for javascript and css optimization - I'm still looking for a solution. I've tried supercharge but I'm afraid it breaks a little too much...