I must admit I was surprised this wasn't a standard transition in Vegas, after using it in Premiere 4.2 around 10 years ago, including the ability to soften wipes.
It can be done easily in Vegas by purchasing Pixelan Spicemaster, or there may be other plug-ins or a workaround within Vegas using Parent/Child tracks - not sure about this though.
There's a quick description at the bottom of this page on how to do this in Vegas with any gradient, including moving video. http://www.rchv.com/store/detail?tp01