It'll all depend on the software. There wasn't a strong lineup for the end of 2008 and most of 2009, which is why Wii sales were down. But once games like Wii Fit +, Wii Sports Resort, and NSMB Wii came at the end of 2009, sales picked up again to near 2008 levels. Looking at the software releases so far, I think Wii can most definetly pick back up again to near 2008 levels, though I don't think it'll reach record heights anymore. Games like Galaxy 2, Metroid Other M, Monster Hunter 3, Red Steel 2, Epic Mickey, etc, will keep the Wii doing strong numbers, as well as the continued strength of NSMB Wii.
Vitality sensor is the wild card. It could ultimately skyrocket Wii sales to unprecedented heights, or it could be a dud and have little to no effect on sales. Either way, it's going to come down to the strength of the game library to carry the system.