Well. I think you should switch the vbulletin default stock smilies. They aren't really the type of emoticons that blend in on your board so a switch would be a wise move imo.
Transparency is always a good thing for images when implementing them to a background or any for that matter because you don't have to worry about excess/redundant edging as long as the image is static and it can be saved in that format (.png) but looking back at my history and I have included many emoticons with animation so I can't save to .png and have to resort to .gif which has trouble displaying in some browsers so a matte option for me has always been a good choice to counteract this grievance. In anycase alot of designers choose to opt for anti aliasing and this makes things difficult when saving to full transparency for animated smilies and the save for web/matte options works well and envolves more work when making modifications for people that request.
Anyway, to answer your question. I find the vbulletin smilies better than other boards (except wtcbb because I designed them

)because I'm not a fan of pixel orientated emoticons which other board software choose to use as their default.