[Rowan]

C2H4 + 2Cl -> C2H4Cl2
C2H4Cl2 + ? -> C2H3Cl + HCl

What is it that separates the HCl from the 1,2-Dichloroethene?
Why don't both Cl molecules get detached?

Any help would be much appreciated!
Thanks

[Hunter2]

Probably NaOH.

[orgopete]

Heat, see http://www.pvc.org/en/p/vinyl-chloride-monomer-vcm.

[Enthalpy]

[...] Why don't both Cl molecules get detached?

I can interpret your question in two ways:

Why not abstract both Cl atoms? That is, as Cl₂. An answer is that this process is energetically worse than abstracting HCl, because two Cl atoms bind weakly, weaker than Cl with H or C. Some bond dissociation energies there, sources differ:
http://www.chemicalforums.com/index.php?topic=80085.msg296423#msg296423
You can also imagine that if adding Cl₂ across a double bond is easy, the reverse operation, abstracting Cl₂ to create a double bond, is difficult.

Why not abstract two HCl molecules? The first one costs 71kJ at 298K, the second towards acetylene 98kJ, and both operations involve the same number of molecules. If RT equals 6kJ, the 4.5*RT more make a strong difference. I's also conceivable that the process removes vinyl chloride from the reactor as it forms.