Yes and no. No in that it will not act as a "transltator" between Poser and Vue. For example, I'll explain what Propack does with Lightwave. You create a Poser animation within Poser with any characters, etc. that you want. You then open Lightwave and import the Poser animation into it, where you create the rest of the scene as you wish. (Adding buildings, streets, trees, hotdog stands, whatever.) The characters will actually be INSIDE of this animation, not merely on top (in front) of it. Big difference. A panel appears inside Lightwave that allows me to adjust (edit) the Poser animation to suit my scene. For example, if everything looks great except one of the characters walks "through" a hotdog stand instead of around it I can directly access the Poser animation from within this panel and alter the character path. Vue will do this, but does not use Propack to do it. This is where the "Mover" add-on for Vue comes in. ($99.00) It will allow you to communicate between your Vue scene and Poser and allow them to work together, again with your animated characters actually INSIDE of your Vue landscape. With Bryce, (and am almost 100% positive also Mojoworld), etc. your Poser animation would appear in front of your Bryce, etc background. Not the same thing. Change your camera angle once and the perspective will be completely wrong. Your characters will never be able to actually enter a house, walk between trees, walk down an alley, stand behind a BBQ, etc. Always in front, and don't move that camera. (Shadows won't match etc.)
What the Propack WILL do for you is allow you to "bone" your own characters and props which will allow you to animate them. Great feature. To create your own characters, however, you will need a modelling program, of which many are free but that's another story. Propack also comes with additional ready to go characters, as well as multiple viewports which makes it alot easier to to work.
So, to recap: To make belivable animations with your Poserguys actually carrying on inside of a believable animated world you need Poser4, Vue, and Mover. You simply will not be able to to create believable animations for less money than with this combination or without a severe learning curve for a higher end app. The Propack is a nice extra, and will allow you to upgrade to Poser 5 inexpensively when they fix it, but isn't necessary. For the price of Propack you could pay for Mover AND buy the Brycetech characterpack with over 100 Poser-ready characters, and you can make believable animations until your computer burns out. I'm 99.9% sure that Mover does not require Propack, but check. Phillipe?