The vacancy clause of Article II is the exception, not the rule. It is not designed to be an end run around the duty of the Senate to vet the President's nominees.
The Senate being in session was a sham.
Also, Presidents do not have the power to declare when a house in another branch is in recess as Obam did, or to adjourn Congress for a day to place his cronies in power. These would be clear violations of the separation of powers.
But, he will almost certainly have four votes in Scotus, so as usual, the outcome will be a cr@pshoot.
That's the argument. The counterargument is that everybody has the power of simple observation. Is Congress "in session" when no quorum is present, and hasn't been for weeks? I know Congress SAYS it is in session, but is it in fact, in session?
The court can duck the issue completely, if it wants to.
I think that Senate failure to grant an up or down vote to a nominee is a violation of separation of powers, as it gives a minority in the Senate the power to effectively reject a president's nominee (without voting to reject) by causing the body to go into a state of inaction. No Court would touch the Senate's dysfunction with a ten foot pole. It's a political issue. The court would reason that it is up to the people to deal with the issue, not up to the court.