Yuuzhun Vong Vs. Imperium

[Norade]

The best bet that the Vong have is grabbing some uninhabited or otherwise primitive systems and then they become just more Xenos for the IoM to deal with. At most they can, upon arrive take a handful of systems and if they come close enough to say something like the Tau they could possibly take a few more world. That said they should forget about ever becoming any more powerful than that. Things like Orks ad Nids out breed them, the IoM and Chaos have numbers on them, Necrons likely won't even be effected by the Vong weapons, Tau are small enough that, with a foothold, the Vog could beat them, and finally the Eldar and Dark Eldar depending on when they meet, might actually have trouble especially when the DE try to torture them and they get nothing back.

[takemeout_totheblack]

So the Vong can run away real good and maybe perform a few hit and run attacks once they find out just how fucking huge the Imperium is. Of course that's no 'invasion' in any sense, but it's pretty much the only way I can fathom the Vong even lasting the year in the 40k Milky Way!
They can't park themselves in some 'quiet' part of the galaxy and bolster their numbers to any reliable effect what which the Orks, Chaos Gods, possibly a Tyranid swarm, Eldar, Dark Eldar, Tau, and all the accumulated horrors of the GrimDark future baring down on them. So all they can really do is run away until they die through slow attrition or quick brutal slaughter. Their lack of psychic presence may hamper active hunting by any one party, but that galaxy is so full of horrors that odds are that they WILL bump into something possibly more powerful and certainly more numerous than them, and that that something will want to make them suffer quite a bit before they die. In the end the Vong are in a place where a swift and horrible single-stroke extermination is the best they can hope for.

So in the end the Vong die: significantly faster FTL and more powerful ship weapons (I think, they use Relativistic weapons or some kinetic thing right?) simply cannot make up for fewer people, fewer ships, tech vulnerable to commonplace weaponry (virus bombs), and the fact that in the GrimDark of the 40k universe there is only war, a war that handles threats like the Vong practically on a daily basis with fairly little effort.

[lance]

Meh, its with in their capacity so we can't discount it.

True. You might even explain the fact that they never use a conventional BDZ with their strategic needs - much like you would do with the Rebel Alliance.

However, we have seen them doing BDZ, with a method that is highly impractical - placing a lifeform on the planet (which requires defenses) which then pulls an orbiting moon (which needs a moon) onto the planet - over a course of weeks (which leads to the question why they just don't use a ship).

The moon dropping was done before open conflict, so that is a reason there. Did they ever go back to the planet? I seem to recall either the moon drop planet or ithor got colonized.

You could argue that this is a great tactic if you want to be sneaky - but it is totally unusable against any IoM-world.
Furthermore, we never see a normal BDZ despite their willingness to destroy worlds - so it might be possible that they are either incapable or unwilling to do it.

Was their ever a need?

As for speed, they are faster than KOTOR era Mandalorions. Don't know how that compares.

I think people are over estimating how bad their ships are, dying really is. The previous leader thought they could reach another galaxy, and a couple generations isn't a lot for a race that has traveled for several milenia. On the lines of your dog living to 15 as opposed to 14 years.
As for dying dogs, my dying german shepard, was starving due to health issues, could still beat the crap out of my much younger Newfoundland. It was really funny to watch.