In case it helps anyone, I think maybe one thing I can do is just not to shut Vegas down.
When I finish with the basic project, and have all four renders done (Internet HD, SD, Blu-Ray and DVD), then I can save the project as the next (English Subtitles) version. Then I can add my new subtitles, and do all four renders on that as well.
Then I can do the Spanish Subtitles version, and etcetera. That is going to be a serious pain, because I have to wait on my translator, and hopefully I won’t lose all of the additionally generated media.
I can use Photoshop, but I think I would have to use track motion to position the media, which would mean about a bazillion tracks. I would think that would get very unwieldy.
Anyhow, I hope this helps someone.