Thanks Firemansam for quick reply!
As I need to re-clear security I will go for 2+ hours connection.
Of course I'd prefer to book this itinerary on one ticket, however VN doesn't sell connecting flight from DAD to PQC on their website. I can only do it with some GDS's, however the price difference between two separate tickets bought on VN website and one ticket with some unknown to me GDS (with rather low rating and bad reviews) is more than double. Even if I miss my not-secured connection it would be cheaper for me to buy a new ticket for the SGN-PQC leg already at the SGN airport than to buy a single ticket for entire journey up-front.