What's the easiest way to add a watermark?

I have a Sony X500V camcorder that produces .m2ts files. I usually the accompany Sony Playmemories s/w to join files together, but now I need to overlay a watermark on top of the enjoined file.

I have Sony Vegas, but something is screwy in that my new file is jagged/slightly blurry. Is there a better program to place a watermark over a .m2ts file that is a series of joined files?

Thanks.