In full Phoenix mode he is intangible - well attacks bypass him so he has some sort of intangibility.
It should be assumed because Vergo is a vice admiral & vice admiral power level are far inferior to Admiral level which Marco is commonly believed to be for the right reasons. Doflamingo thought his right hand man had a superior enough haki to not be fazed by Law's slashing powers. As Law already said Doflamingo is overconfident and was practically basing Vergo wining on a beating that happened in Law's past.
I knew someone would say that. I know you do not believe the Admirals to be that weak that they have to team up against anyone apart from a Yonko or someone on that level.
Why would a prominent figure be scared of another prominent figure unless it is a Yonko level character or someone like Akainu because Akainu would have just killed him no two ways about it. Everybody knows Kuzans character all he was doing was preventing Don from harming Smoker further. Kuzan was prepared for a fight. Don walked away from it. Even Don said he needed Smoker to be quite as he knew too much, we all know now that it is vital information that he would kill to keep on the down low but he could not do anything with Kuzan there so he said he would have to find other means to keep Smoker quite. Doflamingo crew probably know him best, his crew mate was seriously concerned when he got frozen & was relieved when the ice did not freeze his heart yet.
Yes he also showed concern with Issho because the intel Don gathered on the 2 new Admirals was that the were beastly in terms of power & this was confirmed when Issho casually brought down a meteor.
Issho was never after Don, I believe he decided to attack them both because Caesar kept calling him Joker an infamous underground broker. Obviously words were discussed off panel & Law was Issho's target but Issho was still does not trust Don. Don constantly is trying to get Issho on side also.
Marco has better showing against people of Admiral level. Marco got distracted & double teamed that is the only reason he sustained any injury at all during the war.
We do not know if Ace had it or not, he probably did but as a logia never needed it/trained it.
Ace has infinite fire power at his disposal. It will take too much effort for Law or Don to put him down + this is a team battle Marco is not going to sit there & let them pick on Ace.