The Virtual Boy flopped because Nintendo hated it, they just released it to recover some of the lost money invested on it and then moved on.
Nintendo KNEW the DD just... was not going to work due to costs. And they knew that very well after looking at how Sega did with the 32x and to a lesser extend with the Sega CD.
Worse, the extra 4 mb of ram of the expansion pack just ended working better on the Nintendo 64 that in the DD.
There was no real incentive to use the DD as it didn't even use CDs or at least a custom disc format like the Sega Dreamcast did.
The DD was one the the biggest mistakes Nintendo did with the Nintendo 64, but most likely didn't hurt sales as much releasing two years later that the Sega Saturn and Sony Playstation did.
Those two years really helped Sony a whole lot, not only Playstation games got better and with faster load times,;but Sony released new revisions of the Playstation that failed a whole lot less.
Oh and the Playstation also got Crash Bandicoot.