ahhhh, kind of ambitious don't you think. You should know this if you are planning on making it. You can't just pick up a language and make it. www.4guysfromrolla.com has some stuff on site statistics tracking. If you want to service that many sites you will use very little bandwidth, because the ods are you are not going to host a page, you are going to have them put a little form or some js on their site or something to pass the values back to your server, that is the way I have seen other counters done. You could use an iframe, but iframes are going to be invalid html, most people do not want iframes on their sites either.
VB.NET and C# are both good, it just depends on which one's syntax you are more comfortable with. I like vb.net more, but someone else would liek c# more, though we can do the same stuff with about the same amount of code.