I tried the code below. It will give you the complete execution log. From this I just parsed my required log. It's not the best way but I was not able to find anything else.
//log the Vacuum command information
NpgsqlEventLog.Level = LogLevel.Debug;
NpgsqlEventLog.LogName = VacuumLogFilePath + "rolling.log";
NpgsqlEventLog.EchoMessages = false;
try
{
//Run the Vacuum Command
NpgsqlCommand comm = new NpgsqlCommand("VACUUM VERBOSE ANALYZE", connection);
comm.ExecuteNonQuery();
}