Giles, Platinum is a version of Vegas Movie Studio, which is the mickey mouse version. Vegas Pro is the more fully-featured one they have, and 9.0c is the current version. I'm still on 8.0c because 9.0a/9.0b had a number of issues, and there haven't been enough new features in 9 to warrant an upgrade in my personal case.
The Sony YUV codec is not very compressed at all. File size is bigger than Cineform AVI for example, which is a low-compression codec intended for editing. Sony YUV is really not intended for uploading to YouTube etc..
In Vegas Pro 9 there are YouTube upload templates. There is an SD one and an HD one. I'm pretty sure they are using the Sony AVC (H.264) codec so in Vegas Pro you find them in the list of templates (file > render as > save as type > sony AVC > ...). I don't know if they are also in Movie Studio. HOWEVER, since a week or two ago, YouTube are doing 1080p so if you've got a decent connection then you want to be uploading 1080p to them. The Vegas Pro YouTube HD template is 720p. If you choose to go for 1080p you'll probably want to double the bitrate from the 720p template.
When you encode in Vegas, make sure you have a deinterlace method set in the project properties before you render. I tend to use "blend". If you don't have a deinterlace method set then you can get crappy results if there is any resizing going on.
If you want to try Vegas Pro 9, do yourself a favour and download the free trial which (as I recall) is fully functional. They have fixed a lot of stuff recently in the 9.0b and 9.0c versions so a "cough" version may well bring you trouble.
There are better formats for uploading to YouTube which can be reached by frameserving to Megui or Virtualdub and using third party open source codecs (my old SD method is here and my more recent 720p method is halfway down this thread). But they're very convoluted really. You can probably just achieve the same by throwing a bit more bitrate at the standard Vegas templates.