After having set the US strategy in the first round of lunar exploration missions, inspired by its "project Red Socks", JPL decided to get involved in the next round also.
To this end it started studying a plan, in October 1958, which included both lunar and planetary exploration. The plan was called "Vega" and was approved in January 1959 after the apparently successful Soviet Luna-1 mission. As initially designed, the plan envisaged the development of a new "mid weight" launch vehicle, called Atlas-Vega. It was to consist of the Atlas ICBM as the first stage, the first stage of the Vanguard launcher as the second stage and a new hydrazine-nitrogen tetroxide third stage developed by JPL itself.
Many missions were then scheduled for the new launcher: an experimental meteorological satellite, an experimental geosynchronous telecommunication satellite, a two man space capsule, a 600 kg Mars probe, to be launched in October 1960, a similar lunar probe, to be launched in December 1960 and a Venus probe to be launched in February 1961. The project mainly aimed at giving the US a space leadership over the Soviets.
Many factors were conjuring against Vega: the paucity of funds, as NASA had just approved the Mercury project, aiming at launching men into space; NASA naivety, as the agency believed to be able to attain a launch rate of one Atlas-Vega every two months, while the launcher had never been tested before and the launch pad was still non existent. The major problem, however, was not a technical one but a management one: the military were neither involved nor interested in the launcher and were in fact secretly developing a launcher with similar characteristics, the Atlas-Agena B.
In the meanwhile, the first Vega launch began moving towards 1961, thus effectively canceling the launch of the Mars probe. The Venus probe was then delayed until 1962, as nobody wanted to launch it on the very first Atlas-Vega. By moving the timing of the project, the development of Atlas-Vega started conflicting with the Atlas-Centaur development. It was then decided to cancel the last two Atlas-Vega and to completely overhaul the project which now consisted of two meteorological satellites and four lunar probes. 
The mission of the lunar probes was never decided in detail but one of the more interesting was an orbiter which was to take pictures of the lunar surface from a 800 km high orbit using a photographic system similar to the Soviet one used for Luna-3. As an alternative, Atlas-Vega could have launched a 200 kg soft lander or a 300 kg hard lander.
In december 1959, eleven months after the start of the project, NASA had to cancel Atlas-Vega and to adopt Atlas-Agena B. It is worth noting that the cancellation of the JPL built third stage meant the end of the laboratory's association with rocket propulsion, which remains to this day in its name.
Just one month later JPL proposed a new program of lunar probes using Atlas-Agena B and technology being developed for the Vega probes. This program was called Ranger.

Click here to see an image of the proposed Vega probe flying by Mars
 

Bibliography
Ertel, I. D., Morse, M. L.: The Apollo Spacecraft: A Chronology Vol.I, Washington, NASA. (available on-line)
Ezell, E. C., Ezell, L. N.: On Mars: Exploration of the Red Planet 1958-1978, Washington,NASA. (available on-line)
Koppes, C: JPL and the American Space Program, Yale University Press, p. 102-105
NASA Kills Vega, Adopts USAF Agena, Aviation Week, December 21, 1959, pp. 18-19
Vega Study Shows Early NASA Problems, Aviation Week, June 27, 1960, pp. 62-68

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