Also, in Telegram, when you select the chat, it's focuses on the input field, and you can type messages immediately. But in VKontakte you have to focus on the input field manually

Sound notifications are very important, cause there's no point in sidebar messengers if you have to have a messenger page opened to get notifications.
As for the second part - I think all features realized in one supported messenger should be in the others as well, because it's a bit confusing.

English isn't my native language, so I'm apologize if there are some mistakes