I personally think that it is quite unnecessary to prepare much for the TOEFL exams. You can brush up on the basics yourself using a review book, without the help of teachers. But if you feel you need somebody to drive you to perform, there are countless of TOEFL coaches in town.
The first one that comes to my mind is Mr. Nghiêm. Well, he's the only one I studied with, and then only for a very short period of time, so I can't really compare him to other teachers. Another very popular teacher is Mr. Mạnh. He has a reputation of being extremely strict but very thorough in his class. I think the one in Láng Hạ St. that you mentioned is Mrs. Vân. Lastly, there are numerous English centers which employ foreign teachers but are relatively expensive.. If you want to contact one of those, just send me a private message and I'll see if I can help.
On a side-note, even if you don't want to take the TOEFL or improve your English in general, I think you should attend some of Mr. Nghiêm's classes. You may gain some fresh, interesting perspectives - which are never superfluous, no?