I have the same issue, couldn't get Strong Regenerate. Was told that vendor inventory refreshes and updates itself to scale to your level every time you level up. I believe this is true. Now every time i level, i will make an effort to visit all vendors, even if i'm mid way in dungeon.
Btw for Whirlwind, i got 3 from the Commander in Cyseal, one book after each level up from levels 4-6.
Okay, I had too much time on my hands and went back to an earlier save, then rerolled the inventory for Aureus in Cysael a dozen times at each of levels 4, 5, and 6.
Neither Whirlwind(2), Dust Devil(7), or Phoenix Dive(8) showed up in any of the tests, while every other man-at-arms skill from the general skill level range showed up many times.
OTOH, I have gotten Strong Regenerate(7) frequently through rerolling the inventory. I am tempted to ask, "are you DAMN sure that it was three Whirlwinds you got" because of how it conflicts with my results, but you probably wouldn't have said so if you weren't sure. (Though I do wonder a bit at what party setup you are running to have a use for three man-at-arms melee AOE characters.
(The inventory of vendors is not updated when you level up, but when you talk to a vendor for the first time after having leveled up, so it is trivial to test these things by saving immediately before talking to a vendor for the first time after leveling up).