IDbConnection is the correct way. Instead of having to variables DbConnectionMySql and DbConnectionMsSql, you'll use only one:
Dim _Connection as IDbConnection
.
When the connection object is created, you still have to distinguish:
If (frmMain.SQLServerType = "mysql") Then
_Connection = new MySqlConnection()
Else
_Connection = new SqlClient.SqlConnection()
EndIf
By the way, I'd create an enum for the database types.