A while ago a bunch of people were talking about how some groups are able to get 100+ weapons at Chaos Vellum whereas others struggle to get even 10 between a party, but I don't think anyone ever explained quite how it works:
Chaos Vellum and Von Bon/Hard Magnus/Empress Cygnus (I didn't check any others before I quit) drop rates are split evenly between every party member who is active in the boss fight. Active meaning having inflicted any damage onto the boss at least once during the fight. If you have a party of 6 with 1 person on 20x drop rate and everyone else on 6x drop, and every party member attacked the boss at least once, the drop rate will average out to ((20 + 6 + 6 + 6 + 6 + 6) / 6) = 8.3x. That was my general set up, I'd gift every party member a 2x drop card and have everyone cast a 1.6x familiar and we'd go on 2x weekends, and it worked out reasonably well for the most part, until I was able to solo.
If only one person attacks, the boss drop rate will be 100% of their own drop rate, rather than it being split between the party. So if your main attacker has 22x drop rate and everyone else is on 1x, but only the main attacker does damage to Vellum (basically if they do a full solo) the drop rate would be (22x / 1) = 22x. If everyone attacked though, it'd be ((22 + 1 + 1 + 1 + 1 + 1) / 6) = 4.5x, which is a huge amount lower, and that's why a lot of people were getting very few drops, especially considering Vellum's drop rate is pretty bad to begin with.
If you bring other characters into the main fight map but they do not attack, and they just sit in a corner and either wait the fight out or die and then don't respawn, they won't count as 'active' players, it's generally safer to leave them in the entrance map but it won't affect the drop rate at all if you bring them in. Attacking just once is enough to be considered an 'active' player though, even things like Kanna's teleport which inflicts damage, or the 2nd job I/L teleport passive.
It doesn't have anything to do with aggro from my observations. It's generally an easier fight if the aggro is 'stuck' to the main attacker though.
Also, Black and White Star Potions both work at Chaos Root Abyss. Black Star Potions are +5% chance to find a Unique tier item upon killing a boss and White Star Potions are +25% chance to find an Epic item I believe. They're worth using if you have them but not really worth going out of your way to buy.
Saw a thread on Basilmarket and most people had some vague idea how it worked but they were still a bit off, so here you go.
Something else semi related which as far as I'm aware is already pretty well known, but I may as well post here anyway, anything that gives +%drop rate stacks with other sources (+15% or 20% item drop rate potential, +30% drop rate on Holy Symbol, +30%(?) drop rate on Showdown, etc), but other things that give 2x drop rate like the 2x cards/events multiply, I believe the 1.6x familiars are multipliers too rather than 160% but I don't know for sure.
If you have 200% drop rate gear + 30% HS + 15% IA and you're on a 2x card, wolf underling and 2x event, you'd be on (2.45 * 2 * 2 * 2) = 19.6x drop rate.