You can make a changes to MAPIx VBO as below and send email accordingly:
For High Priority/Importance update the below line in the code
,MAPIMessage.Priority.IMPORTANCE_HIGH at below location :
Using msg As New BPMAPIMessage()
If Not msg.Create(mapi,MAPIMessage.Priority.IMPORTANCE_HIGH) Then Throw New MAPIException( _
"Failed to create message")
For low priority /Importance change HIGH to LOW in above code.