If he was anyone else, Vash would've been worried over his knees and arms a long time ago - they were taking too much brunt as it was, either by dodging narrowly (geeze, Allen was fast) or blocking every hit he could... Which he did. In a way that made it look more like he was falling around then actually fighting; happy 'accidents' that swiftly negated almost every attack of the exorcist's.
Except... the one time... it didn't. The Stampede's attention had been forced between ducking beneath a kick, not running into the wall, and not running into the invention--- thingy.
Normally, under any other circumstance, this would've been a no-brainer. But it was just different, sparring; there was no pressing need to be perfect in every move, every counterattack, every block, every step.
So he'd gotten a bit lax. And he... stepped right into that invention thing. And he'd fallen. Rather hard, on his back, partly wondering what he'd just ran into and partly wondering why he didn't feel like scrambling out of the way like normal. ... Hm...