I posted a question in the official forums and got a great response (
here). I thought this could go into the wiki somewhere.
The gist is that not all recipes are available from alignment vendors (Hero/Villain vendors):
To quote the above reply:
Availability is based on Reward Merit cost.
-Items that cost less than 100 Reward Merits are not available for A-Merit purchase.
-Items that cost 100-199 Reward Merits cost 1 A-Merit.
-Items that cost 200-299 Reward Merits cost 2 A-Merits.
-Items that cost 300 or more Reward Merits do not exist, but would presumably follow the same pattern.
-Items that cannot be purchased with Reward Merits have a special pricing scheme (purples, PVP IOs, Respecs, costume parts, etc).
Because most uncommons cost less than 100 Reward Merits, they aren't available as A-Merit purchases. However, at least one Uncommon costs over 100 Reward Merits: The Steadfast Protection: Resist/Defense, and it is indeed available for A-Merit purchase.