1. Yes you do have to relight. Render Studio works with Vue 6 Esprit, Vue 6 Pro Studio, Vue 6 Infinite or Vue 6 Extreme.

2. That really depends on what you're looking for. When it comes to figures, Poser still does an excellent job and I still prefer Poser over Vue in that area. It's not a matter which one is better, it's a matter what they were designed to do. It also depends on the look and feel you want your renders to have. You want ultra-realism..... Vue does the job. You want more of a 3D comic look.... Poser will do a better job imo. Close up figures or studio like renders.... Poser would still be my #1 choice.

3. I've found the learning curve pretty easy to climb. I had much more trouble figuring out Bryce then I had with figuring out Vue. It's pretty straightforward and everything is in the place you would expect it to be.

*Most of my scenes are interiors or studio work & not outdoor shots.  Should I be looking elsewhere to boost render quality than a terrain program?  What do you advise?

I seriously would not advice Vue for that usage at all. Poser is very well designed for jobs like that. I'm not sure how long you've been using Poser, but judging from your joining date it may not have been too long. Most likely, If poser isn't giving you the results you want, Vue won't give them either. While the rendering engine may be better by itself, you still need to know how to use it at best. The same goes for Poser, for the kind of work you decribe the Poser rendering engine is a very good choice, but you need to learn how to use it and that takes time.

Switching applications most likely will not give you the miracle you're looking for, time, effort, patience and experimenting will. Correct me if I'm wrong here, don't be offended please.

My advice would be, don't abandon Poser yet. Secondly I would suggest checking out Travelers Lightwork series at RDNA. The links below will lead you to some of them. These lightsets are very very well done and they give great results and even greater results if you use them as a starting point for your scenes. That would be my advice, check them out and perhaps buy a few sets and see how they will improve your Poser renders, instead of investing heavily into Vue, you may not get the results you're looking for at all.

http://www.runtimedna.com/mod/bcs/index.php?ViewProduct=2871
http://www.runtimedna.com/mod/bcs/index.php?ViewProduct=3367
http://www.runtimedna.com/mod/bcs/index.php?ViewProduct=3190
http://www.runtimedna.com/mod/bcs/index.php?ViewProduct=3173

There are many more of them, this is just a random selection, browse throught Traveler's store http://www.runtimedna.com/mod/bcs/index.php?vendor=Traveler to find more.

Btw.... one addition. I'm not sure if you bought Render Studio for Poser or not, but imo the  Lightworks sets still beat Render Studio.

Artwork and 3DToons items, create the perfect place for you toon and other figures!

http://www.renderosity.com/mod/bcs/index.php?vendor=23722

Due to the childish TOS changes, I'm not allowed to link to my other products outside of Rendo anymore :(

Food for thought.....
https://www.youtube.com/watch?v=pYZw0dfLmLk