Mountain Dew MDX was launched last November (and reviewed here after a lot of beta testing). To recap, this was Pepsi's attempt to position a hybrid soda/energy drink on the market via the Mountain Dew brand, probably in an attempt to steal a little of Coke's thunder with their launch of Vault.
Unfortunately, Pepsi sort of botched the rollout of this beverage. The biggest drawback was the size and pricepoint. While the majority of soft drinks these days are sold in 20 oz. bottles at gas stations and convenience stores, Pepsi chose to put Mountain Dew MDX in a smaller 14 oz. bottle AND charge more for the privilege. At the time, Pepsi proclaimed that "the launch of MDX also marks the debut of a new and unique single-serve 14-oz PET bottle. MDX's exclusive packaging is convenient, portable and features a re-sealable cap and easy-to-grip bottle." Aren't most plastic pop bottles convenient, portable, re-sealable, and easy-to-grip? Predictably, the beverage has not sold very well.
On the flip side, Coke seems to have a marginal hit with Vault, which must put a little fear into Pepsi's eyes, given their long dominance with Mountain Dew. Industry sources have reported that it's doing pretty well, with Coke supporting it via more marketing efforts. Vault Zero came out late last year as the diet version of the drink.
Sensing their mistake, or maybe just a little fearful of the growth seen in the Vault brand, Pepsi is relaunching Mountain Dew MDX in a standard 20 oz. bottle at a lower pricepoint. Duh! Nothing else about the drink has changed. It still has a caffeine level of 5.875 mg per ounce, just like Vault (which is higher than the 4.5 mg of caffeine per ounce found in standard Mountain Dew).
The bottle is the same as used for other Pepsi 20 oz. drinks. Packaging maintains the same logo and branding, proclaiming the infamous "Power Pack" of junk inside. Probably the only change is the increased use of the phrases "Energy Soda" or "Energy Infused Soda" in more prominent locations, again, I would assume, to battle Vault's marketing line of "drinks like a soda, kicks like an energy drink."
Personally, I don't think MDX has a chance. It has had virtually no marketing behind it aside from the launch and has very little buzz. The Sugar Free version of MDX is pretty much no where to be found, while Vault Zero has good shelf presence. The original MDX website no longer seems to be live. And overall, Vault continues to grow in visibility both in pop culture and on store shelves.