The 'Like' feature is built into the vBulletin forum software so there isn't the option for a Dislike button at this time. It may be added to a future version but nothing is on the cards for now.
I can understand why people would want to have a 'dislike' button but I think its a bit 'hit and run' negative. I think we should all be positive or constructive/polite in our responses, personally. I think a dislike button could be taken badly and cause ill feeling to the recipient.