I have solved the problem using the Youtube API. The problem was the way the "video" object is created. Here is the working code for people that need it:
Dim commentingSettings As New YouTubeRequestSettings(appName, developerKey, Token)
Dim lcommentingRequest As New YouTubeRequest(commentingSettings)
Dim videodUrl As New Uri(String.Format("{0}/{1}",Google.GData.YouTube.YouTubeQuery.DefaultVideoUri, "BtrFZmZjalE"))
Dim video As Google.YouTube.Video = request.Retrieve(Of Google.YouTube.Video)(videoEdUrl)
Dim comm As New Comment
comm.Content = "comm text here"
lcommentingRequest.AddComment(video, comm)
Now the code will work!
The new question is: How do I get the comment ID after I posted? Is there a way to get it from the code above (some server response or something)? ... but as Chris M. suggested I will create another question for this...