The fruit was incompatible with the devil sword Sparda.
Why? Because it was the sword that was used to defeat anyone who consumed the fruit to begin with.
Vergil had already split himself in half. Urizen most likely feared further damage to himself if he tried to bond both the Yamato and Sparda together the same way Dante did.
The reason why Dante and Vergil waited to stab themselves in front of the portrait was because:
Vergil knew he might lose control of his demon half and would forever remain a human until his death.
Dante in his sin trigger form, could barely move around. It's the same with Urizen. Both of them -- while very powerful -- could barely move and keep their sanity intact.
Vergil's sin devil trigger could move very well, but he had to rip himself apart and put himself back together to be able to have that much control over his body. And even then, he still lost against the combined efforts of Dante and Nero.
"You must be both strong in mind and body to wield the Sparda."
Urizen lost some of his memories (as he told Dante) and Dante, after fusing the Rebellion and Sparda together, was driven to kill Vergil, and hated himself for it.
That's why the power was off in the beginning of the game.
Dante barely had the will to pay off the bill.
As for Vergil, he lamented everything he had done when he was V, the manifestation of his human guilt.
That guilt only intensified when V became fully Vergil again.
It was the reason why he waited for Dante to attack first, because he knew having that much power would fuel Dante's rage enough to make the first move.
Vergil, instead of killing Dante right then and there, delayed the inevitable.
Despite his guilt, Vergil wanted to see how powerful he had really become, and if he really could stand against the power of both Rebellion and the Sparda that manifested itself in the devil sword Dante.
He succeeded. He would have ended Dante in DMC5 had Nero not intervened.
Nero's power came from his experience training in the Order of the Sword, fighting against Dante and Sanctus, fighting (and losing) against Urizen twice, and refusing to let either Dante and Vergil die, despite the fact that it was Vergil who ripped off his arm.
Sparda and the Rebellion fused together were extremely powerful. Powerful enough to take down Urizen after he consumed the fruit of the Qliphoth.
However - Yamato, Nero's power upgrades he added to the Yamato via his arm in DMC4, the Qliphoth fruit, V's deceased monsters (Nightmare, Griffon, and Shadow), and even V's Cane (Checkmate) almost defeated sin trigger Dante.
However, Vergil's combined power wasn't enough against the devil sword Dante, the devil trigger-powered Red Queen, and Nero's newly formed "Spirit Yamato" (Showdown).
All of these things combined were just enough to stop Vergil temporarily, even though he said he could "still fight".
Which is exactly why Vergil said, "I won't lose next time."
Because he won't.
