As I understand it, the form bonus applies against damage you receive by your casting action, but not if you cast a damaging spell on yourself. The latter would be a matter of the form of the spell.
When do I receive damage by my casting action? Maybe ritual magic, when my spell total falls very short. But then I receive wounds instead of a damage-total..
Any explanation?
The only example I can think of would be a spellcasting botch. Ordinarily, I think a GM would just inflict a wound, but it is certainly conceivable that they would instead come up with a damage total (perhaps level/2) and ask the magus to Soak it.
Soaking damage in a Disaster when Experimenting?
Using the virtues Life Boost or Life-linked Spontaneous Magic it is possible to push yourself beyond consciousness and start taking damage. In those circumstances, the vim form bonus makes a difference.
thanks, that explains it..