It has inferior audio tools by comparison. If 70% of video is audio than having to purchase Logic and learn a new interface is needed, just to do with what Vegas does out of the box.
Vegas has better compositing tools. Neither Vegas nor FCP have the capability of Boris Red or Adobe After Effects. The tools in Vegas will carry many people a long way.
Sony Vegas is the most intuitive and easiest program to use. This translates in productivity.
I will give FCP kudos for multi tasking, but I for one when I am editing am focused on that task. I do not need to surf the net and chat while editing.
Sony Vegas functions well on an older and more modest machine and functions fantastic on a newer capable machine. So when you "upgrade" to FCP it is likely the machine you have will not function.
FCP needs a video acclerator card where as vegas does not. The advantage is that you have real time viewing of effects, the disadvantage is that it adds another thousand dollar cost for the card, in addition to the thousand dollar Logic for sound editing. Vegas works without the video accl card has near real time review of effects and works fine for me.
I am not at all saying that FCP is a bad program, it is a great expensive program, but when compared to Sony Vegas, it cost thousands more, is more difficult to use and is inferior in capability. It does however work on a Mac and if you have Mac it is well worth considering.