This witcher contract can be started at the Notice Board in Oreton, a village in the western half of Velen. From the notice board you must pick up the note that says "Contract: Missing Soliders".
Talk to the commander in the village and travel to Byways further south. Here you must kill 5 ghouls which is the hardest part of the quest. After they are dead you talk to the villagers and follow some clues to reach the catacombs below the village. Here you must fight Sarasti, a Vampire boss. It's a relatively easy fight. When you attack with quick strikes it will stagger Sarasti, giving him no chance to attack you. He has relatively low health too, so this fight shouldn't last long.

Talk to the villagers after slaying the monster to end the quest. This will also unlock the Fearless Vampire Slayer trophy / achievement.