It should be very obvious, but it doesn't seem to work that way because of the bazillion icons one is presented with when replying to a posting, there's no understanding of hierarchy. You need to be looking at the end of the second row of icons. This is the "quote" button. You need to understand that the buttons don't do anything magical, they simply insert text surrounded by square brackets. Then, you need to have some text in your copy buffer to insert between the sets of brackets. If you can7t work out how to use the icons, you can simply type what you want to say. For example:
I broke wind prodigiously
It must be the beans, he thought.
Call100 has it exactly right: Call100 does exactly the right steps to quote, but doesn't recognise that those steps are correct, because of the way vBulletin presents the option.