I'm similar with you.
September 2017 Application
Med request 05.Feb
Med pass 15.Feb
Then a long silent till now. It's almost 3 months after Med. So worry and anxious. I'm from VN too. Many other people in same month of Sept Apolication both get their pre-arraival email, even thet done Med after me. Tell me if you get news.
You should not be worried. Why are you? I am sure you know that an outland application could take up 12 month for DM if not longer!
There is no rule that says you can compare the applications together. Some applications take less then 6 month some 8 month and some more, we don't know what will happen to the application once it has been submitted. Some VOs are very very busy with PR applications hence the long wait! Some VOs are not busy like Ankara(Turkey) and really quiet and with so many officers, so they process the PR applications faster than any other VOs.
I know that London VO is unimaginably busy, as the receive applications not only from UK and Ireland, but also from so many other countries such as Denmark, Finland, Sweden, Pakistan, India, Afghanistan, etc..., so this will explain the long wait.
My Med was submitted in early November 2017 and I am still waiting! I don't think that you should be worried. I have learned that applicants can't do anything but to wait.
September applicants must be very patient as it seems like the process is really slow for them.
You may wanna share your timeline from ECAS.
God speed.