This is a vBulletin/Browser Bug involving the way the QUOTE blocks are styled. Because they use divs, which are block-level elements, 100% width of the container is calculated. Only inline elements will wrap to floated objects (like the text-node inside the QUOTE block), but the box containing the text will not. This is a quirk of CSS (and isn't fixable by setting display: inline) and is not solvable unless vBulletin restyles the QUOTE block using inline elements (such as SPAN) or a TABLE (which would not be proper use of the TABLE tag, but which only stretches to the available space).
Even if certain wiki elements like IMAGE blocks were recoded with inline elements, this problem would still persist because it's the way div, hr, h2, and other block elements interact with floats.
Instead of restyling the QUOTE block, another option is to "clear" it, so that if there are floated objects around it, it will render either before or after the float, rather than in the same line.