In order to remove the predefined http://  string from the Insert Link functionality in the vBulletin text editor open the clientscript/vbulletin_textedit.js file and change the following code excerpts:

{url=this.show_prompt(vbphrase.enter_link_url,"http://",true)} (appears twice)

and

{this.prompt_link("url",url,vbphrase.enter_link_url,"http://")}

to

{url=this.show_prompt(vbphrase.enter_link_url,"",true)} (appears twice)

and

{this.prompt_link("url",url,vbphrase.enter_link_url,"")}

Also, navigate to your vBulletin admin panel->Languages & Phrases->Search in Phrases. Search for the enter_link_url phrase in Phrase Text and Phrase Variable Name. Edit it and make sure that http:// is not present in the default text window.