I'm not sure how helpful green tea is for vertigo and migraines but I did look up some useful herbs for you.
For vertigo: Blessed Thistle, Gingko, Rosemary, and Wood Betony are excellent healers. Wood Betony is also good for head and brain circulation... which it is said that poor circulation is the most common reason for head aches.
For migraines: Feverfew and Lavender are good.
These are herbs that my reference book has listed. If you are thinking about taking herbals for medicinal purposes, I suggest researching each herb thoroughly. Some herbs have side effects and cautions (like some should not be taken if you have high blood pressure, etc). Consulting a physician is a good idea, as well.
As for making the teas taste better. My book said that normally half a tea spoon can be mixed in with other herbal or flavored teas to make a 'blend.' I have also seen pre-made blends of 5-10 herbs in tea bags already.
I hope this helps!