It's true one can wonder why the Gemini spacewalkers didn't feel motion sickness, while outside of the cramped spacecraft. It might be bacause the EVA didn't last long enough. Or maybe, because the helmet prevented the astronaut from turning his head.
Borman was sick inside Apollo but not inside Gemini. The room inside must have a lot to do with motion sickness.
Frank De Winne, while inside the Soyuz on its way to ISS, last november, was told not to look outside until they got inside ISS (because the capsule manoeuvred to orient the solar pannels towards the sun and to get closer to ISS, which regularly upsets cosmonauts).

Also, when astronauts train in the vomit comet, it seems very hard to pinpoint who's going to get SAS (space adaptation syndrome) and who's not. I 've read Scweickart never had problems in the vomit comet, but did , once in space. I do regret he didn't get to spacewalk from the LM to the CM, but only to stand on the porch (well, it was far better than nothing).

Raoul