VB5 doesn't even have a responsive admincp style. The admincp is coded using frames. The use of frames is deprecated in HTML5. Using the admincp from a mobile device is probably not a common thing anyway. If you really have a need for administrating a forum from a phone, this should get you started on writing your new responsive admincp interface. http://www.w3schools.com/html/html_responsive.asp
Simply no need for a responsive admincp, not like you're browsing the site, posting, and such as a normal front-end user you simply visit to do whatever administrator actions you wish at said time then you're done. If one is spending hours within admincp for some odd reason then maybe but only then and even then it's just overkill as who else will be utilizing this? Anyone, Anyone?