When you are in Vet Ranks, you no longer get 0 XP from mobs that are more than 5 levels below you, but you do get less than you do from mobs within 4 VRs. So a VR1 mob will give you standard XP when you are VR1-VR4, and less once you are VR5+. I'm sure the percentages are on here somewhere, but I can't find them right now.
Edit: Found it. Not sure if these numbers are still accurate, but as of 1.5.4, it was this:
You will receive full XP credit for monsters 1-3 ranks below you.
You will receive 75% of XP for monsters 4 ranks below you.
You will receive 30% of XP for monsters 5 ranks or more below you.
Well me at vet 9, a vet 8 and vet 11 ran it with a vet 1 and still got loot. I went maybe a 20th of my xp bar.
Got some really good loot from too.
Main thing is levelling undaunted. Started running 1 light and 1 heavy piece so need that undaunted mettle passive big time! That went up almost half a bar so that was good