How do the Dark Eldar and the Necrons travel within the Screaming Vortex?

The DE normally use the Webway, so does this mean that there are Webway links active within the Screaming Vortex?

As for the Necrons, I have heard that they can use the Webway (per recent fluff) and that they have developed realspace FTL from Inertialess Drives and/or artificial wormholes. For the Webway, this is the same question as for the DE. For the other options (realspace FTL) I ask first if it's still part of their currect fluff that they have these options, and secondly how would these operate within the Screaming Vortex (which is not entirely realspace)?

The Nexus of Shadows (ref: The Soul Reaver, pg 9; Rogue Trader adventure supplement) is "hidden within the webway" and well-located for raids into "the nearby Koronus Expanse, the Calixis Sector, and even the Screaming Vortex...".

As for the Necrons, they are known to use teleportation as a means to regularly traverse great distances- there may be such a teleportation node located within the Black Catacombs of The Pillars of Eternity (Tome of Fate, pg 108), perhaps? Other (normal) means of entering the Screaming Vortex are the 13th Station of Passage, Exile, and The Harrowing (Tome of Fate, pg 74, Inside the Screaming Vortex). Also on pg 74 is a summary of the methods and ways of travel within the Screaming Vortex as a whole.

Being 60+ billion years advanced, one might assume Necron technology is sufficiently sophisticated as to make travel within even warp-intruded space a non-issue (tachyons, and other such).

I thought that warp rifts (like the eye of terror and the screaming vortex) only came into existence with the fall of the eldar which happened way after the necrons went to sleep, so they may not have experience in navigating such areas.

From what I understand (look into Battlefleet Gothic...), normal space travel for Necrons involve using their pyramids and smaller crescent shaped starfighters. I'll read up more on it, but so you know, there is a pdf on the GW website detailing the Necron fleet (specialist games<battlefleet gothic<battlefleet gothic resources<Necron and Tyranid Fleets)

General "fluff" indicates Necrons do not travel the Warp at all. At least not as a real-space alternative. There is the 5th Ed Mat Ward garbage about Dolmen Gates breaching the webway : http://wh40k.lexicanum.com/wiki/Dolmen_Gate#.UdWTrztJNWI
Inertialess Drives, if I remember correctly, reduce mass to theoretical negative weights (typical scifi mumbo-jumbo, really, though based loosely upon current studies of atomic structure theory) in order to travel great distances between point A and point B at FTL speed. But how much FTL? A little? A lot? Instantaneously?

Instead of "Inertialess" Drives, I refer to Necron (and related) drive technology as Relativity Drives, or Singularity Drives (start here: http://en.wikipedia.org/wiki/Event_horizon ).

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A Necron vessel utilizes an extremely detailed and complex calculation engine known as a galaxy orrery to plot their trajectory through real space from point A to point B. In simple terms, it's like drawing two dots on the same face of a piece of paper, then moving the dots in a straight line toward each other so that both dots connect...but without folding the paper. Further to this, the remainder of the page's open space (both front and back) moves, as if you were folding the paper- this pseudo-represents planets and other celestial phenomena as they move about within their solar systems; the orrery calculates this movement, engages the Singularity Drives, resulting in near-instantaneous travel, quite literally in the same amount of time it would take you to fold a piece of paper.
All celestial phenomena follow complex physical laws, some understood, others...not so much. The Necrons plotted a vast majority of these physical laws (gravitational fields; planets getting smaller, thus lighter, by infinitesimal proportions due to radiation shear; etc) post-War In Heaven, much like the Mayans calculated their "doomsday" calendar by plotting the course of our own solar system's celestial bodies, except the Necrons didn't go only so far and call it good. They constructed the galaxy orreries, which are perpetual calculation engines, plotting the births and deaths of stars, their attendant solar systems, and the mobility of their celestial phenomena in perpetuity. The end result being the Necrons don't collide with anything while moving from point A to point B.

EDIT: Necrons very rarely ever collide with celestial phenomena. However, there are plenty of void tales of Imperial vessels having departed upon a journey never to be seen again, and it is quite possible (every once in a very great while) these ships are simply obliterated as they are struck by the FTL mass of a Necron vessel (which is also obliterated).

However, the galaxy orreries are not infallible. As I said, the Necrons deduced a vast majority of physical laws, yet not all of them. And then there is the Warp. Necron travel is a lot FTL, but longer distances must be broken down into a number of individual travel segments- plotting around gravity well twins or triplets, skirting the changing boundaries of Warp storms such as the Eye of Terror, Storm of the Emperor's Wrath, Van Grothe's Rapidity, the Screaming Vortex, and others.

There was a Star Trek "online" game waaaay back in the mid to late 80s. You piloted your vessel by engaging thrust, and chose direction by pointing and clicking your cursor in the desired quadrant. There was a brief hiccup as you clicked, the briefest fraction of a moment before your vessel changed course...this is all the longer it takes the galaxy orreries to plot and calculate each travel segment. Eastern Fringe to Halo Stars in approximately thirty seconds. Not quite teleportation, but close enough as to be indistinguishable.

As for Necron travel within the "close" void (ship-to-ship combat, planetary orbit)? "Antique" mass reaction engines.