Hey Mouse42.
I also dread fly season! I don't have any experience with garlic and yeast supplements for fly control. I've seen the ads in catalogs for garlic and been very curious to know if they work. Anecdotally, I eat lots of garlic

and mosquitoes never seem to even notice. Maybe the supplements have perfected some combination?
On an earth-friendly note, I did start using a non-chemical fly spray this summer. I'm sort of holding my breath as fly season really gets going to see if it will actually work though.
I've never tried Vaseline for the ventral area you mentioned. My horse also gets pretty raw and itchy there in the summer. Would Vaseline make it hard for that area to "breathe"?
Let us know if you find more info on the effectiveness of garlic/yeast supplements in your research.
[I'll also create a new forum for Horse Care -- the Bulletin Board does need one.]