Fantastische Hure said:
Considering to his knowledge he had an hour left, it should have left him with enough time to test to see if he could over-whelm him as easily as you say he could.
Vegetto said he didn't plan on prolonging it at all, so it makes sense for him to be going all out even if unnecessary. After all, he'd already seen Zamasu's immortality take attacks from SSG Goku despite being at best close to his SS3 form's power, so it makes sense for Vegetto to not be giving a stronger Zamasu any chances.
SSJG Vegeta was said to be a joke by Goku-Black so either you can go by what you said. Or Vegeta managed to switch in and out of SSJB in such short bursts that he could dodge when he saw something coming.
Black's opinion was based on Vegeta telling him it was a form lower than the one Black previously defeated. Vegeta wouldn't be able to switch in and out in the first place if Black was above SSG Vegeta by enough to blitz him.