Trojan.Agent refers to a generic Trojan malware detection. It is used for detections that are either associated with no specific malware families or not enough information is available to pinpoint the malware family.
Trojan.Vilsel is the detection for a family of Trojans that:
Malwarebytes protects users from Trojan.Vilsel by using real-time protection.
Malwarebytes can detect and remove Trojan.Vilsel without further user interaction.