Namek and moons

[Majicka]

Vegeta goes from calm but slightly scuffed up to vein-foreheaded and sweating when he summons the ball and throws it in the manga.

He's also going "huff huff" in multiple panels after having summoned the ball - PRIOR to the beginning of his transformation, suggesting it took a lot of effort.

In the english translation (I have #2 of the big anthologies), Goku says "H-his power went down as soon as he made that weird light...!!"

My only problem with that is that for the entire fight, everyone's power is drastically going down. At the start of the fight when Vegeta blows up the plateau he is knocked into, he has his veins and stuff showing, but he appears to be just as strong as ever.

There's several times in that fight where people do attacks that show that they take a lot of energy, yet after a few moments, they are back in their usual stride. That's a recurring theme through all of DBZ.