I've used both for a long, long time. Here are some general observations on the subject.

E-on Software has probably the closest working relationship to the Poser creative team of any 3rd party "3D rendering" software. They have direct import support for Poser scene files, including geometry, textures, materials and animation. (but not lights or cameras) While the geometry import is usually 100% accurate (including exact Poser scaling) the materials import does not fully and accurately support all of Poser's material nodes, nor would Poser materials necessarily look better than Vue "native" materials even if it did. (this is particularly true of Vue metals, glasses, liquids and special FX materials) I usually replace all Poser materials with Vue materials if there ARE corresponding Vue materials and no specifically important texture maps involved. Even if I don't swap materials, I usually tweak many of the parameters of the imported materials. 

For stock Poser human figures, I consider "Skin-Vue" to be an absolutely necessary plug-in. I can't stress enough how much better is makes Poser figures look.

In all honesty, the Poser FireFly render engine can't be compared to Vue's. Vue has 4 atmospheric models (regular, volumetric, spectral, and HDRI), 4 rendering strategies (raytrace, Global Ambiance, Global Illumination, and Radiosity)  at least 7 types of lighting, tons of shading options and more kinds of anti-aliasing strategies than I can remember. This, of course, makes it hard to quantify Vue's render speed. Do you want a good looking render in minutes? Vue can do that. Do you want a masterpiece that takes days to render? Vue can do that too. It is without doubt the single most flexible render engine I've ever seen.

The "outdoor Vs indoor" issue is bogus. Those who say Vue is best at outdoor rendering and you should use another program for indoor rendering simply do not know how to use Vue's render engine well enough. If done properly, you'll match anything coming out of Max, Maya, or C4D no matter what the setting.

One last thing I will bring up. I've never gotten trans-mapped hair to look quite as good in Vue as it does in Poser. I consider this to be a failing on my part rather than Vue's. I'll work it out some time but I'd love it if the maker of Skin-Vue could create a "Hair-Vue" so I don't have to.

I'd rather stay in my lane than lay in my stain!