First off, the post is obsolete, from last November. This was during efforts to balance things during the time when VP and XP were standardized to 1M XP/VP per vet level pre 1.5. Some settings were bugged. None of that applies post-1.6, since the XP scheme has been continuously tuned since then.
Second, XP has NEVER been contribution based. The loot has been, but never the XP. AP is another story.
3rd, I believe that the XP level step-down goes like this: full xp at level to 3 levels below. 1/2 XP at 4 levels, 1/4 XP at 5 below, and nothing for 6 or more levels below. Note that all veteran ranks are actually level 50, so you will get XP for all vet kills. The sliding scale starts at 50, for vet players. A vet player will not get any XP for kills at level 44 or below, which I confirmed in my test. Now, I just spent an hour testing this out on a Vet 14 character. The lowest level I tried was level 10. I got no XP, but some hide loot.
The XP earned in Vet levels is a bit more complicated, but there does appear to be a step down there as well, though I wouldn't swear by it. Think through what you would need to do to test this out empirically, with some completeness. Makes me shiver.
I think that the comprehensive treatment you're looking for doesn't exist. I agree, tho, that it should, eventually. However they are still balancing the game after the introduction of CP, and another tuning will take place once the Vet levels go away. Bottom line is, this stuff is constantly changing anyway. On top of all of that, if you're a subscriber, you get a 10% boost. And then there's XP boosters coming... Oh, and did I mention Mara's Rings? Never mind.
No final answers here, but I hope this helps somehow.