[Carlos Silva] "a) Considering images generated previously, could just the animation be done directly in Vegas?"
Yes, The easiest way to add animation would be to use animated GIF files. These will play back in Vegas and then you could use Track Motion to move them around the screen. Many animated GIFs like the butterflies and flying Pegasus in your video examples can be found on the internet. Just drop them on the timeline, resize them with Pan/Crop and preposition them with Track Motion.
[Carlos Silva] "b) If yes, what you told about the use of Vegas Pro because “masking”, would it be applied as well? Or Vegas Movie Studio is able to do that?"
You wouldn't need to use masking with files like animated GIFs because they have an alpha channel and will mask themselves.
~jr
http://www.johnrofrano.com
http://www.vasst.com 
