VB Mario Land is a cancelled Mario game that was originally going to be released on the Virtual Boy.
VB Mario Land was to be a two-dimensional game that would also have three-dimensional elements in it and would star Mario as it's main character; though not much about VB Mario Land's story was revealed, though it had Mario battling Wario for some unknown reason.
VB Mario Land would make use of all the Virtual Boy's capabilities, with Mario being able to enter the background of various levels; additionally, dungeons, similar to the ones seen in The Legend of Zelda series of games were also prevalent in VB Mario Land.
Also in VB Mario Land, a three-dimensional version of Mario Bros. was mentioned as being implemented in VB Mario Land, though aside from it's presence in the game, no more details about this Mario Bros. minigame were mentioned.
It is possible that several elements of VB Mario Land were implemented in Mario Clash, most noticeably it's Mario Bros. elements.