in my forum when 500-700 user become online then forum not loading i see cpu use 1% max 180mb ram use i have 2gb dedicated ram how to fix it.
700 users is not really a whole lot for making the forum slow. 2gb ram should be more then enough to handle that. there are many options to speed up the forum. first of all see if compression is enabled, if not you wanna do that. also see if your javascripts are on page code or file. if on page see if you can put them on a file. that will make it load once and reuse it on next requests.
Trying using a better caching method like xcache, clean down your plugins and make sure you remove any you no longer use or need, then go through your files on the server to make sure there isnt any old plugin files left on the server which no longer need to be there ( you can do this with the vbulletin suspect file tool in the admincp ) , you'd be surprised what old redundant files do to the server sometimes as i found out not long ago.