VGChartz data is not reliable. They claim to get data from retailers, but they will not say how many data points they have or which retailers they use. They retroactively adjust their figures when data from companies such as NPD, Media Create and GfK Chart-Track is released, and they have been out by over 100% in the past. One example is the game, Iron Man, which they said had sold 53,000 units in a month. NPD released data showing that it had sold 130,000 units. VGChartz then adjusted their data to 111,000 units.
You can be quite accurate using only a relatively small amount of data, but VGChartz have shown many times that their estimates have been way out, which to me makes them pretty worthless as a source. For long term sales figures they are more accurate due to them adjusting their figures to match other sources, but for future predictions, I wouldn't pay much attention to their numbers.