Velah side chest is very high level loot I think it's among the highest loot level chests in the game
good at business
Axer bta upgrade tomes were confirmed by feather to be the same as an unbound tome but there's a 25% chance any time you pull is converted into a bta upgrade to the next highest number. They are in fact unseverable from one another in the loot table.
If someone pulled a +4-5 upgrade in that chest as reported that means they looted a +4 unbound tome and hit the 25% chance to turn it into an upgrade tome.
good at business
BTA or BTC +4 tomes can become +4->+5 upgrades if they come from the standard tome table. The only time they won't upgrade is when they come from a raid specific table w/o upgrades in it. The only confirmed (by Rodrak) table like that I know of is the extra CITW roll for tomes/tomes of fate/purple cubes. (That's the extra roll only - you get rolls from the standard loot table and can get +4/upgrade from those)
Not the case, i've seen reports of upgrade tomes in nearly every raid, even cases where they do not meet the minimum loot level.
When feather said he added them everywhere else they used to drop as non upgrades, he wasn't kidding.
EG: Shroud can drop +3 to 4 upgrades now too.. Shroud certainly does not hit the required loot level 23, but it does drop +3 BTC drops. (on end rewards)
So if VoN6 epic chest drops +4 unbounds, it's an indication that it has a loot level27+. The fact it drops upgrade tomes tells us nothing as we can't determine which table they come from easily. If you got both a piece of raid loot, and a +4 to 5 upgrade tome, that could be an indicator, but we can't be sure someone didnt just assign you extra loot or something else, so a unbound +4 would be better proof.