I am planning on starting a wordpress blog on root (mydomain.com) and vBulletin forums on a subdomain (forums.mydomain.com). Both are tech related, but on blog I will be posting tech news, updates and all other good stuff (especially windows and troubleshooting) and on forum
it will be about how to fix windows software and hardware issue and all other geeky stuff.

Now my question is, should I use same or different keywords for both domain and subdomain? Also, if you have any other suggestions regarding this configuration then let me know before I start :)