I agree that Vita and 3DS sold similarly in America pre-price cut. Look at VGChartz's numbers for weeks 5 through 19 for both consoles (3DS on left, Vita right):
Week 20 was the price cut week. Sure, 3DS wound up with a considerably larger install base after 19 weeks, but that was mostly due to its stronger launch and occassional big week (see week 13 at 31k, that's Ocarina 3D week). Week-to-week, their sales were generally quite close.