The VEI scale might be the most ill-begotten piece of quasi-physics ever devised in the history of mankind. One thing becomes abundantly clear and that is that Chris Newhall and Stephen Self might be good volcanologists, but they are not in any way physicists.
Let us start with a basic problem. Eyjafjallajökull was a VEI-4 calculated on primarily the amount of tephra ejected, and secondarily on the height of the ash-plume. Grimsvötn ejected as much tephra, and had an even higher ash-plume. Both are VEI-4s according to the Volcanic Explosivity Index (VEI).
The basic requirements for a VEI-4 is that it ejects between 0.1 to 1.0 cubic kilometer of tephra, and has an ash-plume that is anywhere between 10 and 25 km high. Eyjafjallajökull ejected 0.25 cubic kilometers of tephra and had a peak ash-column of 9 kilometers. Grimsvötn is not fully tallied up, but it released the same amount of tephra in its first 24 hours, and had an ash-column that was 20 kilometers high. Eyjafjallajökull took 60 days to erupt the tephra, and only for a couple of days had a 9 km ash-column. But let us say that they both released the same amount of energy during the mentioned time spans, just for arguments sake (at least for now).
What is an explosion? It is the almost instant release of pent up energy. 1 quarter kilo stick of dynamite is the same amount of energy as 2 Mega-joules of energy, or the power equivalent of a 550 watt travelling hairdryer running for 1 hour. Guess which will blow up your car?
And here we hit the head of the VEI-nail with a stupendous physics hammer. If we go back to an explosion being your basic energy release over a specific time, then the time issue gets rather critical quickly. Why is this important? Well, the energy release over the specific time frame decides how destructive an explosion is. Dynamite is destructive, a hair dryer is not destructive (unless your stylist is a moron).
Now some of you will have tallied up things and come up with a huge difference between the eruptions. If you are a normal sane person you have now calculated the difference in destructive force between Eyjafjallajökull and Grimsvötn being 60 times larger. With sane I mean that you did not study physics. To loft up the same amount of tephra in a sixtieth of the time poor Grimsvötn needed to use Eyjafjallajökulls total energy squared. This is actually simplified; Grimsvötn erupted through a 16 times wider muzzle and lofted the load to twice the height. For those of you who own guns, you know what I am talking about. But in the end the number of 240 times more destructive will suffice. (Dear colleagues, I am simplifying things here.)
To go back to the analogue, Eyjafjallajökull was not a travelling hairdryer compared to Grimsvötn, it was more like a professional hairdryer used for half an hour. It will still though not blow up your car; it will just make your hair look un-natural quicker.
As you by now know, all VEIs are not equal, and sometimes physics is good for tearing down idiotic scales created by people who just want to have a scale named after them. When I have the time I will create a scale that actually measures how destructive an explosive eruption roughly is, but that would still not take into account all sorts of destructive forces involved in an eruption. It would just be a small component of the problem, because in the end the non-explosive Laki eruption killed more people than any other volcano in the history of mankind. VEI, yeah right! I think I will acronym my poor formulation into Destructive Index of Eruptions.