Well, I'm going to be honest with you: I have no idea, and the information we have is so poor I guess it's impossible to come out with an accurate estimate of the death toll.
Personally, I'd go with a WWI-like T-V War, and a WWII-like Great War, as it makes more sense to me. First, because I can hardly imagine an all out war in space that doesn't involve great empires lasting so long, and second, because I believe the Shivans completely changed the rules when they first appeared in FS1, thus leading the PVE to end its hostilities with the GTA.
By the way, in our history we've had a war that lasted en entire century, mostly because it had its on and off periods that made it possible.
EDIT: Also, speaking of planetary bombardments during the T-V War, they would have broken many rules of engagement had they not been aimed specifically at military installations and bases on the surface. I can't speak for the Vasudans, but the Terrans sure would have made a big deal of the unnecessary death of millions of Vasudan civilians.
On the one hand, mjn.mixael is right in that it's ultimately up to the campaign author. There's no clear canon information.
On the other hand, if I were writing the campaign I'd go with what Mobius is saying: an on-off war, with brief flurries of activity followed by stand-off periods. For one thing, if the war had been going full-bore for 14 years I'm pretty sure one side would have collapsed a while back. For another, neither the Terrans nor Vasudans seem to have been trying to exterminate the other race; it was probably more of a competition of planetary resources.
Personally, I think 15 million (that's a million a year) seems too high. An Orion carries about 10,000 personnel, and a Typhon likely carries a similar crew. Cruisers have much smaller crews, apparently in the hundreds but we can't be sure, certainly much less than the 6,000 crew of a corvette. 15 million is the equivalent of ten or eleven destroyers lost per year, and it's unlikely that that many destroyers were lost over the whole war
. It would probably take at least 5 cruisers (guesstimating) to equal the crew of a destroyer, and given that destroyer losses were rare (probably less than five for the whole war), that means each side is loosing 50-60 cruisers a year, which seems unsustainable. Pilot losses are to be counted too, but given that it's one pilot a ship, and more then 20 pilots a sortie is very rare (esp on the terran side), they really wouldn't count for much compared to capship losses, and they certainly wouldn't make much progress towards a million a year.
Also, loosing 500,000 people a year for 14 years seems like something that would severely hinder the "Alliance", which seems to have formed very quickly after the Shivans appeared. While I'm sure there were people angry about it (ST:R) losses on that level would likely have lead to much more rampant speceisism.
Lastly, both species have a pretty clear concept of ROE, and seem willing to apply these rules to other species, so I doubt civilian casualties would be very high. Orbital bombardment with Harbingers would have been a sort of MAD atrocity.