[nerbert]

A problem I see here is that the browser still transmits the password with the md5 hashes and an eavesdropper can still intercept them and crack them. All your advanced hashing happens on the server after it receives the md5 hashes. What you could do is replace the original vbulletin_md5.js file with one that does the md5 and bcrypt hashing . Then the password is better encrypted in transit. I think you would have to use vBulletin's original code and then add more code (available as open source) to further encrypt both versions of the hashes. Once this is done you could skip the PHP code that bcrypt hashes on the server

There's a further thing to consider though, it seems to me that there's a problem with vbulletin_md5.js and older (and maybe newer) versions of IE, so it doesn't encrypt at all. You'll have to check this out with various browsers. There's open source md5 javascript available and you could copy the vbulletin code that handles the difference between vb_login_md5password and vb_login_md5password_utf into the new md5 algorithm.