I also do exactly as stated before, and I also have 8GB. It's a very convenient and fast workflow, once you get used to it.

Regarding the question of render in Poser or Vue, there's no doubt that Vue is much better. Your idea of rendering the background in Vue and exporting it to Poser has many problems. Just the complication of making the light in Poser to exactly match the light in the background, this alone is a sufficiently strong reason not to do it. It will always look fake, unless you work a lot on each image just for this purpose. Wasted time. And many, many scenes you just couldn't do in this way. I don't want to advertise my images, but take just a look at my latest image and think how this image could be done in this way, I mean prepare the background in Vue and render in Poser. I would say it's very very difficult because the environment around the characters is not just a background, it's an environment, all around the characters. Link here:
http://www.renderosity.com/mod/gallery/index.php?image_id=1806409

Depending on the complexity of your scenes, I would really recommend getting more RAM and getting a 64 bit system. You will not regret it.

But all this above depends very much on what kind of scenes you want to make. If all you want to do is portraits, I would say you really don't need Vue all that much. For portraits, IMO, Vue is a nice-to-have, not a must-have. For portraits only, you could use Poser or, maybe even better, Daz Studio.