I would like to open projects (.veg files) I made in Vegas 2.0 in Vegas 3.0 so I can render them again using the new encoder in VV3. Will it allow me to do this?
Yes, this will work fine. However, if you save your projects from Vegas 3 then you won't be able to open that .veg file in version 2 anymore. I would suggest you make backup copies of the .veg files just to be safe.