The history of this game might be a bit confusing and makes it necessary to have a closer look at its background first. Practically, VB Mario Land is a prototype of Mario Clash. Mario Clash was once a Jump'N'Run with adventure elements, which is the now unreleased VB Mario Land, but this prototype also contained a mini game, a 3D version of the classic Mario Bros. game, which was later released as a stand-alone game called Mario Clash. So the correct title of this game should be "Mario Clash", but to make it less confusing, it is widely known as "VB Mario Land".
The game is very similar to the first Super Mario Bros. on the NES, it features a similar levels structure and even has a small castle and a flagpole, which we all know from Super Mario Bros., at the end of each stage. Like in VB Wario Land, the levels consists of two layers, on which you move through the stages filled with enemies and other obstacles. At some points, bridges allow the player to change between the fore- and the background, but you can also move between the two layers through pipes. What makes the game special, besides that it is 3-dimensional of course, is the combination of platform and adventure elements. So, for example, the pipes lead to Zelda-like dungeons, which let you control Mario from a top-down point of view.
Like already mentioned, a 3-dimensional remake of the classic Mario Bros. was implemented as a mini game. It is unknown though, how it was implemented exactly, maybe you could make some extra points in it between stages, or it was a seperate bonus game, like in the Super Mario Advance games.
An early 1 level video of VB Mario Land was shown on Winter CES 1995 and looked very promising.