VIENNA - Heightened security measures have been ordered at embassies and other diplomatic buildings in Vienna after a soldier stationed outside the residence of the Iranian ambassador to Austria shot dead a man who tried to stab him, police said on Monday.The 26-year-old assailant's motive for the attack, which took place around 11:30 p.m. (2230 GMT), was unclear and video footage would be reviewed, a police spokesman said."The attacker stabbed the soldier repeatedly. He was only saved by a stab-proof vest," the spokesman said, adding that the two men ended up grappling on the ground."The soldier defended himself well and was then able to reach for his service weapon and fire shots," he added.