SOAS Mosque on Vietnam stamps

I managed to acquire these stamps recently from a Vietnamese stamp dealer. I did not get these when I was in Vietnam but I saw the stamps on ebay and I was quite intrigued. I have a number of stamps issued by other countries which depicted Brunei's scenes but I have never seen this one before.The stamps were supposedly issued in 1993 and with a face value of 8000 dong which is roughly equal to