I apologize for making contradictions between my interpretation of the rules and the actual rules. I'm trying my best right now to make a good judgement call based on the rules that are written right now.
What bugs me about all of this is the "meta-game" aspect that's going on right now, which is using OOC knowledge to make IC decisions. I know that cannot be avoided, and is justifiable to some extent (if we assumed IC spy networks, informants, double agents, etc.) - but BIG decisions are being made without any way to have possibly known how far along Dementat was in his Diplomacy mission - and so to try and time a perfect attack in the middle of a PDF swop-out is a little precarious.
So far, Dementat has followed the rules of the Diplomacy mission precisely. He did the required actions of Milestone 3, which was to build a Fleet Depot and replace the Space PDF, and he did so PRIOR to knowing about any Rebel invasion because he began construction of those two VSD-IIs on October 1st!
To void the 2 VSD-IIs would be to void the very mission itself. He's using a Shipyard at Kuat to build everything, so the VSDs completed construction on October 6th, and it takes 3+ days for them to get from Kuat to Eriadu, so essentially they could have been there on October 10th but he was still in the middle of Milestone 2 at that time. Not only were those VSD-IIs ready to go, but they departed as soon as they could to complete the Space PDF replacement action of Milestone 3 immediately.
There will be no ruling against those VSD-IIs. Eriadu's Space PDF has been replaced. Personally I'd prefer taking on two VSD-IIs instead of an Omega, Golan, and Sigma... so if that is the situation you preferred, please do not ask me to make a ruling when from my point of view the only thing that has happened for the Rebels and Republic is REALLY BAD TIMING.