What you ‘re doing is called a nested veg.
As long as it’s Vegas 6 (the full version, not Movie Studio) or newer, import the old veg file just like you would any other asset (video clip, audio clip, image, etc.) and drop it on the timeline.
What you’ll see is a single video track and a stereo audio track, no matter how many tracks the original veg file had.
This veg can be edited by right-clicking on it and selecting “Edit Sopuurce Project”.
This will open the veg in a new instance of Vegas to allow you to make any changes necessary. Once you save and close it, the veg in the project you’re working on will be automatically updated.