Really Graham, you have sometimes a curious way of knitting things together.
Vital comes from the latin vita - life.
Elan means "spirited" or "enthusiastic". It can also refer to someone who is in motion. Elan vital is the main subject of philosophy of Bergson, one of the more wellknown and important French philosophers.
Did you not already suggest life force as a translation? Comes close to elan vital.
Much the same as what I said no? I add vital as a word because it too has the same root and shows that Ki is vital. Vital life energy.
As is prana, qi, chi, etc.