Womens clothes shops stock UK size 16/18

Good luck.     Your plight may be like those males trying to find size 13 (UK 12.5) shoes.  The upside may be that once you get your feet on the ground, and meet a few Vietnamese female friends to help, you may be able to have very nice clothes sewn at very reasonable prices only a little above "off the rack."

Yeah I was thinking get some made! I feel your pain with the shoes....I am a size 9.5 but not worrying too much about that as shoes are not worn within the schoo I am working in.

Try these shops:

1- http://quanaobigsize.com/

Their address:  http://quanaobigsize.com/lien-he/
1263  Hoàng Sa – Tân Bình

There are 26 reviews for an average of 4.2 stars.  If I were you, I would ignore the English translation of customer reviews since it doesn't reflect the original (very flattering) comments.

In the left and right panels, under SẢN PHẨM, you can see what they carry in the shop. 

2- http://shopbigsize.com/
10/1 Cống Lở -  Q. Tân Bình (open daily between 10:00 - 18:00)

3- This website lists 7 of the top famous shops for "big size" in Saigon:
http://sakurafashion.vn/top-7-shop-quan … -4249.html

You may be able to  understand the gist of it.  If not, let me know and I'll translate for you.

There are quite a few more.  If you fail to find something suitable at those shops, I'll dig a bit deeper in my search.