* Santos wants "active" vice president
* Vargas Lleras may boost chances of first-round win
* Election will focus on FARC peace process
By Helen Murphy and Luis Jaime Acosta
BOGOTA, Feb 24 (Reuters) - Colombia's President Juan Manuel Santos on Monday named German Vargas Lleras as his vice presidential running mate, a popular choice that will likely boost his chances of winning a second straight term in office later this year.
Vargas Lleras, a former Santos cabinet minister and senator, will campaign for election in May and promise to help bolster economic growth and steer peace talks with Marxist FARC rebels to end Colombia's half-century war.
Santos is the front-runner by far to win the election in a second round of voting in June, but having the popular Vargas Lleras as running mate raises his chances of clinching a first-round win.
"German Vargas Lleras meets the requirements of vice presidents like few other Colombians," Santos told reporters in Bogota.
"We have cultivated a sincere friendship over the years based on loyalty and honesty ... such virtues are every day more scarce in politics."
A candidate needs more than 50 percent of the ballot to avoid a runoff.
A rival for president in the 2010 election, Vargas Lleras became Santos' key ally in government, helping increase both politicians' approval by handing out free housing to the nation's poorest as housing minister.
He also served as Santos' interior minister.
The 52-year-old lawyer has maintained among the highest approval ratings in opinion polls, raising speculation he would turn against Santos and run for president for the Radical Change party he represented in 2010.
Santos said his vice presidential pick was not based on political calculations but on Vargas Lleras' qualities and experience in guiding key reforms through congress. He called for a "different, more active" vice president.
It is widely expected the center-right Santos will back Vargas Lleras for president in 2018.
"I can contribute so much if I am given a concrete task to fulfill," Vargas Lleras said after the announcement.
Santos and Vargas Lleras will face rival candidate Oscar Ivan Zuluaga, who represents the party formed by former President Alvaro Uribe, a bitter opponent of the peace process which is expected to become the campaign battleground.
Like Santos, Vargas Lleras comes from one of Colombia's most powerful families. He is the grandson of former President Carlos Lleras Restrepo and nephew of former presidential candidate Carlos Lleras de la Fuente.
He holds a doctorate in political science from the Ortega and Gasset Institute of the Complutense University of Madrid and served as president of the Senate.
A one-time ally of Uribe, the two fell out and became bitter rivals following an assassination attempt on Vargas Lleras.
In 2005, he escaped injury when a car bomb exploded as he left a media interview in Bogota. Uribe publicly blamed FARC for the attack, going against an investigation that pointed toward right-wing paramilitary involvement.
He also lost two fingers from his left hand in 2002 when he opened a gift-wrapped book that exploded.
Vargas Lleras' support for the peace process with FARC, that began in Cuba in late 2012, is key for Santos as he has been among the most vocal in backing a hardline stance against the rebel group.
Once an investment pariah, more than a decade of U.S.-backed military offensives against the FARC and right-wing paramilitary drug trafficking groups has helped turn the Andean nation into a magnet for foreign oil and mining investment, boosting economic growth.
But both criminal groups remain active, especially in rural areas, frequently staging attacks on economic infrastructure and threatening opposing politicians. (Reporting by Helen Murphy and Luis Jaime Acosta; Editing by Sophie Hares)