Again, I think the Vita has next to no chance of getting the system unless it is a massive success its first year (think Wii numbers from 2006-2009) because the mainline Dragon Quest games always go on a system that already has a large install base.
That's the pattern the series has taken up to this point, but that doesn't mean the series will
always follow said pattern. Square-Enix has spent too long screwing around with this game, amazing considering how mechanically basic these games always are. By the time this game finally released on Wii, they would be the only major company (including Nintendo) still releasing Wii games. You can't say that IIRC for
any of the previous numbered Dragon Quest games, which is why I think the pattern breaks here.
I think it much more likely that this game goes multiplatform in some capacity, much as its company has over the past few years. Whether that's Wii and Wii U; 360 and PS3; or 3DS and Vita is debatable, but one thing I'm gradually becoming sure of is that this is not going to be
just a Wii game. Square-Enix needs a bigger hit than
just a Wii game that will sell
only in Japan these days, especially with the Wii in the severely weakened state it's in software-wise.