Garage floor build up has XPS on top of Radon membrane and below concrete slab.  Do I need to put a VCL on top of the XPS as well?

What is on top of the XPS?  Is the garage heated / "conditioned"?

The top layer does a few things

a very important one is stopping runny concrete getting under the sheets and floating it up, which would be a bloody nightmare. 
for £20 install it. 

We did above and below - XPS is much better than EPS but still absorbs water a bit and thus loses it’s value as thermal insulation.

Edited June 11Jun 11 by Alan Ambrose

VCL goes at the bottom. Between the insulation and concrete you can put a thin polythene layer, as mentioned above to keep concrete out of the insulation, if it gets in your insulation will float up.