An effective voice-over for a vlog is when the voiceover is well understood, not rushed, and corresponding to the visuals. It must have a natural, expressive, but not too loud and soft voice. To maintain high quality, recording with a good microphone is very important. The tone must relate to the audience on a deeper level..
Good voice-over for vlogs must always have pacing that's in sync with what is being shown in the visuals. If the visuals are to be shown quickly, the narration must be lively too. With that, the video will flow even better.
Realism is another major contributor. Even with good equipment, if the voice sounds robotic, it won't connect with viewers. Adding natural pauses, a friendly tone, and some personality makes a voice-over much more effective.