So you flew back to Da Nang first, where they didn't let you in and asked you to fly elsewhere? Who did you talk to there, Vietnamese immigration or just an airline rep?
I'm surprised that they didn't try to sell you a new overpriced "super urgent" visa for Vietnam, but then again, there doesn't seem to be any consistency in how they handle this.
As for Thailand, you won't have any trouble getting in using a safe entry service. Anytime.