Only VN citizens on repatriation flights, and few foreigners on case by case approval by authorities like experts working on major infrastructure projects etc.
No border opening dates set, and no dates under discussion as per now.
I just cited it from a memory - there is an Ordinance, likely, by Ministry of Interior, where all kinds of extensions listed, long list, incl. medical one, or if someone in court proceedings etc.
only Vietnamese citizens in repatriations flights are allowed in.
among foreigners, then only diplomats, and "high-skilled workers, experts, top managers" basing on individual permissions issued by local UBND along with MFA - actually only those working on grand infrastructure projects and mega factories like Samsung.
many countries have strict requirements about airside transit, e.g. US and NZ would not allow even for airside transit without a visa, so it doesn't look exceptional that other countries impose some restrictions on air-side transit.