Hi Stephen,
Being a new poster on the cow, I may not be able to give you the exact info you are looking for , but, with that being said ive worked with vegas pro 8 9 and now 10 and have a little insight to your problem.
Obviously you need to know a few things: Resoultion of the source footage, If is a HD 1080p then you would want the aproximate preset in vegas.
Also the size in MB or gigs. Some stuff iv’e been working with is MPEG2. If im reading correctly a lot of people have seen long rendering times in different formats. If you say you rendered already, what setting did you use?. This may be a good first step. Maybe this will start a number of posts coming in.