Hi guys I am planning to make a nicely designed site that will have many categories and subcategories and each of these will be filled by user-generated content including text and images (perhaps multimedia too, not sure about that yet). I am well acquainted of HTML and CSS but not much as regards php. So which CMS should I use, Joomla or Drupal? Are there certain features in the description I gave up that would make one CMS preferred to the other , .. or just the general preferences each one of us has?
That sounds like a fairly general site description. I can't think of anything that would make you require one over the other, but my personal preference is always Joomla.
What if i wanted to add vbulletin forums later. should i use drupal because of the vbdrupal thing or i can simply add the vbulletin files and link to the homepage from my joomla homepage?
Drupal has a forum included, but the forum is somewhat basic. The nice thing is that searches and login are all integrated. If you are experienced with vBulletin you might find the Drupal forum a little too basis though. There are modules you can add to the Drupal forum to give it more features, but if you have experience with vBull already, and can afford vBull, you might want to stick with vBull.
Concerning the "user-generated content" thing, would it work with both CMS, or is one of them is preferred as regards that aspect?
It's always better to have common login for CMS and forum. So if you are inclined (rightly so) to use vBulletin as your forum, I suggest to try vbDrupal. If you use Joomla, then there won't be integration between Joomla & vBulletin. You have to maintain them as two separate entities, and that may frustrate your users in the long run. Read below thread once - http://forums.digitalpoint.com/showthread.php?t=633079
You need to be an expert of php to run a CMS site though it would be advantageous for you if you have a working knowledge on the language. My suggestion: use Joomla. It has more features than Drupal. IMHO.