Tiger is worth 5 ENY, Panzer is worth 30 ENY. If you kill a Panzer with a Tiger that would be worth 5 divided by 30 which equals .16667 perks. There is also the "Perk Bonus" value, which is fluid and depends on the difference in numbers between the countries rosters. So lets say when the above kill was made the Tiger's country had the most people on, and the "Perk Bonus" was .79
So:
(5/30)*.79=.131667 perks
Lets say you killed 40 panzers under those conditions. .131667*40= a whopping 5.26 perks.
But you likely didnt kill all panzers, and the higher the ENY value of the vehicle you killed, the less perks you would earn.
Thats basically how it works. It depends on the ENY values, and what your countries perk bonus is at the time.