The swing of mid-tier Japanese 3rd party support still seems evenly divided, possibly even with an edge to Vita, but the Vita's biggest third party titles are currently (much like many of the 3DS' biggest titles have been so far) ports, ports, and ports. FFX, MGS Collection, UMvC3, these will do little more for Vita than, say, OoT3D, SSFIV, and SF64 did for 3DS
We'll need to get a better picture of the original software lineup of Vita before we can effectively make that call, but the problem is while Vita's accumulating that lineup, 3DS sees the heavy hitters emerge, and real, Japan-targeted heavy-hitters at that, and the momentum shall swing