Vasodilation as a "cause" for CH is part of the now defunct theory of "Vascular Headache".
If we had a "cause" for CH, then we would all be closer to that "cure" we seek.
There is no known cause in CH.
The Vascular theory of headache, taught to most current practicing GPs is outdated and no longer applicable, they need to brush up on this:
http://en.wikipedia.org/wiki/Vascular_headache
The Vasoconstrictive effect of Imigran is a side-effect of the drug.
Mean arterial constriction seen in cross-sections of blood vessels, conducted during studies of Imigran is less than 10%. The mechanism of action behind Imigran in CH is still not known, although 5HT Serotonin receptors are implicated.
See: The vascular theory of migraine—a great story wrecked by the facts, by Peter Goadsby.
http://brain.oxfordjournals.org/content/brain/132/1/6.full.pdf
And, Cluster Headache: What has changed since 1999? Here on this site.
http://www.clusterheadaches.com.au/forum_posts.php?id=3071
Magnesium, may help unlock stores of Vitamin D.
Batch's Vitamin D regimen is still gleaning positive results on the US forum.
The Vitamin D regimen is somewhere on this site, under "Medications and treatments".
Perhaps only when sufficient Vitamin D is supplemented, can Magnesium assist as a co-factor in CH relief.