I finally got some time to install the MK3 bed on the Vertex.
And by “installing” I simply mean removing the Velleman-supplied glass-plate, soldering a thermistor on the MK3 as well as the four wires (two power, two thermistor) and putting it on the plate mount on the Vertex with some cork in between for thermal insulation of the bottom side. Extremely easy… 
Note that the height of cork+aluminium is slightly higher than the Vertex glass, so you need to adjust the z-axis (and obviously re-calibrate the bed).
First print (black PLA (Innofil3D), 200 deg hotend, slow, 55 deg bed, 70% flow, glue stick for adhesion):
As can be seen from the image, you loose around 10 mm in the x-axis if you just use the Vertex mount. But it should be easy to gain these 10 mm again by adding a few holders of your own (which is also what I did in the middle).
My external PSU can only handle 4A, so I will have to find some old ATX PSUs to power it. Currently it uses 3.5 A for 55 deg while the fan is running (70 deg without fan) but I like to get it to heat up a little faster. I simply used the current limiter on my external PSU to control the bed, but I have just installed a power expander so everything is controlled by the vertex now.
I bought everything from http://reprap.me. The MK3 bed is €13 which is approximately the same as one piece of BuildTak. 
But then you also need cork and a power expander (or simply a relay or a mosfet of some sort) and a thermistor. The total cost was around €36.
This was my first print without glass, and I must admit I am impressed! I had feared that the aluminium would be non-flat, but so far it seems to be very flat. Note though that the specifications on reprap states +/- 0.2 mm so I may simply have been lucky.
