Sony Vegas 9 Pro Green Screen/ Masking

Here is what I am dealing with. I have a video with me on a green screen, and I have another video of an ostrich (I know this sounds lame just wantin to try something) and what i'm trying to do is, put a mask over the ostrich's head so when my hand goes up behind it I give it bunny rabbit ears. I know how to mask and have masked it perfectly but it either just cuts the birds head out...or leaves the head there and removes everything else. Does anyone have any idea why this is happening?

Thanks for the answer, yes I got that to work but it's really sketchy, because I cant see the ostrich at all and its hard to guess. Is there any way I can do it to make it a lot easier. I'm by no means a pro and appreciate your answer. But is there a way I can make the ostrich show up in the preview where I am at and lower the opacity, or something like that?