It's a little tight to be honest. I arrived on LH772 (in F) a couple of months ago and we got in a bit early. This didn't count for much as the queue at immigration was bad. Loads on the BKK-SGN leg of LH772 are usually light.

They have just (beginning of this month) opened a new international terminal at SGN and the old joint terminal has now been turned over to domestic operations. I haven't been through the new place yet but it is supposed to be quicker to clear immigration. Baggage was always fabulously quick at the old terminal but first reports of the new place were not so. This was during the soft opening though so hopefully it's improved.

Unless it's imperative that you get to Danang on the Sunday night I would recommend spending the night in Saigon and going up the next day. I notice there are no flights after VN328.