Unfortunately I am not an expert on Vegas, but in ULEAD Mediastudio you can customise any transition simply by double clicking the transition on the timeline. Optionally you can add this to a custom library by making your amendments then clicking the ADD button.
You can also purchase additional transitions for mediastudio from a number of vendors including
Crystal Graphics Energize PowerPoint, FrontPage, Outlook & Excel. Power Point Presentation Software, Templates, Animations, Pictures & more or Pixelan Software | Video Effects and Video Transitions (Follow the Spicemaster link) amongst others.
COMPAQ something or other with 500Gb Boot Disk, 1Tb external Firewire Disk, 4Gb Memory and a super duper sound card, 19" TFT Monitor etc etc etc
Sony TRV33e : Sony TRV310e : Canon XM2
Sony Vegas Studio editions to v11
Remember, there is always more to learn than there is to teach.