Our nervous system is Brilliant,what you receive with your brain goes to the entire body,it's all connected,for example if you feel depressed you will see how your voice change,your breathe become shallow,so if you feel down you won't be able to have that great voice during ur vlogs.Even your posture change and all humans subconsciously have the ability to recognize how a person feels just by looking at them.My advice is to do something that makes you happy before the vlog and work to get better in the things you love to do to raise ur self esteem.