I've installed an old skin and it looks like it doesn't support quick reply.
Could there be a possibility to add a quick reply function (just a box and a button) nothing else?Thanks for helping me out here.

lol how old?

save all your edited vbulletin templates to txt files of the same names, then revert them one by one and compare/add the graphical code back in manually.

if you find it breaks something then just put the original saved code back in the template you were editing at the time.

if you are carefull and check the look every time you change a template youll fix it in no time

but first import the original vbulletin-style.xml from your \upload\install folder (make sure the version matches the version of board you are running)

import it as a new style, no parent, any title because it wont show up as a new one it will just refresh any unedited templates with original code and add ones back which were inadvertantly deleted ;).

i cant think of all the relative templates right now except for postbit/postbit_legacy (whichever of those two you are using), showthread, showthread_quickreply and showthread_list are possible culprits

when its all fixed save the style to your hd and youre done