Can Sony Vegas Pro be used or compared to the output produced by Final Cut Pro for professional use?
Vegas is a mature, full-featured NLE. I use it regularly for professional work from spots, to corporate video to feature films. I've also used it for semi-professional things like editing a video of my stepdaughter's choir concert for a Christmas DVD.

Here's where I run into problems. My producing partner is all about FCP. There's no clean way to export an EDL between our two systems so that we can BOTH edit. We work around it. If there's something with Sony's proprietary HD format that's somewhere between HDV and XDCAM, or DSLR footage from a Nikon, I edit because Vegas imports those clips easily. When FCP imports those formats, it ranges from not recognizing the file to displaying interlace artifacts. On the other hand, my Vegas setup chokes on Canon HDV, so that gets edited on FCP.

Whether using FCP or Vegas, the output is comparable, I'm not saying identical because of inherent differences in how MacOS/FCP and Windows/Vegas implement the various output codecs. Needless to say, though, our material from FCP and Vegas has been used in every media from Broadcast to family videos. 

The tool, in and of itself, it not the issue. Professionalism comes from a.) the footage and b.) the skill of the editor. Shoot a corporate video with the onboard mic, no tripod and no lights and Eisenstein himself couldn't make that a professional looking edit. Conversely, light and mic a two year old's dance recital, shoot it with three Steadicams for proper coverage, and you'll get something worthy of a broadcast dance show. 

It's just that for some projects, I need a phillips head and for some I need a flathead. It doesn't matter if Vegas in the flathead or the phillips and FCP the other. Which one is the best for the job? No one's comparing MSPaint to Vegas...it's just a tool, and if you use it right, you'll be very pleased wit the results.