I usually don't watermark in vegas but there are a couple ways to do it:
Add your watermark png to the timeline, I use a large-size png, use event pan/crop to resize and place it.
Remember to change your preview quality to Best-Full to view it or it may look crappy until you render.
or
In photoshop create a project same resolution as your video
1920x1080 for HD
655x480 for SD
with transparent background, add your watermark to the photoshop project, place and size to your taste, save and drop psd into timeline.
