There is no Vergo>>Pica. That's a personal opinion.Let me clear one thing people don't get. Vergo was a beast. Vergo>>Pica.
The flashback showed that Doflamingo met the top executives including Vergo, all at the same time.He was the first Corazon. Was strong enough to kill next Corazon and was longest serving person in Donquixote family. He was the right hand man of Doffy. Doflamingo trusted him enough to take on Law and Luffy both. His strength is noticed by the fact he was able to crush Smoker even after tanking two diable jambe's and countershock.
And the heart seat doesn't mean you're the strongest, take Doflamingo's brother for instance.
Vergo was used to establish and hype FBH. This is Oda telling us of the Donquixote's top executives level.He was still alive and speaking after that cut he got and Law used his devil fruit infused with Haki which is the only reason he won and it pretty much is common sense Law>Zoro. Losing to Vergo isn't a bad deal. Someone like Pica other than that Golem stuff which is too slow to even hit Smoker is not defeating him and other then that Golem Pica has done nothing close to above average impressive. Plus Smoker can fly and isn't it pretty obvious he can sense him and what good that Golem can do to Smoker? Smoker only needs to track and hit the main body not "destroy whole of it". And by the looks of feat Smoker is much better combatant who atleast fought with people rather than gettting one shotted by someone who is below Law and Luffy.
It was to show their toughness, by tanking attacks from Sanji and Smoker.
Just because Smoker and Sanji failed to defeat Vergo, it doesn't mean he's stronger than Pica.
That's like downplaying Zoro's feat, by making Pica look weaker than Vergo, just because Zoro defeated him.
The top executives are Doflamingo's strongest fighters, and Oda doesn't need to show us all of them beating or tanking attacks, from Smoker and Sanji.
By showing Vergo's level and toughness, we can have a little idea of how strong and tough the top executives really are, and their so called opponents.
Usopp, Fujitora and Zoro showed different ways of observation, something that Smoker wasn't shown doing.Btw Ussopp can sense Aura of people miles away. Sensing aura of living person predicting it's move while being blind (fujitora and rayleigh against Luffy) feeling presence is COO. There was no one around Pica who's aura or presence Zoro required to feel.
Just because Oda stated that Zoro's specialty is CoA, it doesn't mean his CoO is weak.It's pretty much clear that around Pica there was nobody and Zoro can't fly or anything but Zoro wasn't even able to detect Pica in castle. The major fail. He was closing eyes again and again and I hoped he'll show some beast COO but it didn't happen. It's clear Sanji is weak in COA and Zoro is weak in COO and both are strong vice versa in haki. Not a bad thing since they both are strong in their own areas and what they want.
You're making a huge assumption there.