In Dragon Ball Heroes anime episode 4, golden oozaru cumber fires a blast against Super Saiyan Blue Vegito and at that time Vegito defuses. Was golden oozaru cumber's blast power what made Super Saiyan Blue Vegito defuse or they are supposed to run out of fusion time exactly in that moment?
The answer to your question is a Yes! However, it isn't for the reason you think it is. It is not possible for a potara fusion to defuse on account of a powerful attack from an opponent (Unless it breaks the potara earrings). This can be easily proved based on a few mentioned facts on the show.
- It was stated that the potara fusion was permanent in case two kais were to fuse and not permanent for mortals. Hence, if a powerful attack were to be used against 2 Gods, they would have to defuse which would contradict the earlier statement.
- In case of Fused Zamasu, which technically should be a permanent fusion, we don't see the characters defuse despite taking a very fatal attack from trunks spirit sword.
You could argue that the reasoning would only apply to a fusion between two mortals.
- When Cumber launched the attack on Vegito, Vegito showed no signs of exhaustion and had absolutely no signs of damage on him. There was no reason for Vegito to not be able to tank that attack, considering the fact that Goku and Vegito alone were able to get up after taking physical attacks from Cumber and Goku was able to push his feet up in his SSJB Form.
- If you also look at Kefla, during her match with Goku, Kefla was substantially exhausted and took a fully charged Kamehameha from Ultra Instinct Omen Goku. Kefla didn't defuse until the magnitude of the attack caused the potara earrings to break. This was UI Omen Goku using his fully charged attack against an exhausted Kefla and that was Cumber using a basic attack against a Vegito who wasn't really exhausted
Hence, the reason why the answer to your question is a yes is because, the more powerful Cumber becomes and launches stronger attacks at Vegito, the more Vegito would have to power up to Keep up and block/counter these attacks. Hence, this would indirectly be responsible for shortening the fusion.