Two plans, one date between them
Right now an apostille for Vietnam is not accepted, so a file going there is closed by embassy or consular legalisation. From 11 September 2026 an apostille takes the place of that step. That leaves you with two honest plans rather than one, and choosing between them is arithmetic about your own deadline rather than a matter of taste.
The first plan runs the consular route today. The second prepares everything now and finishes after the date, when an apostille issued in Birmingham for use in Vietnam becomes the item that ends the job. Neither is clever or foolish by itself; each one is right for a different date at the far end.
What each plan costs you
The first plan costs an extra desk. Document legalisation for Vietnam runs through consular channels until the date, which means another wait and another hand-off once The Legalisation Office has done its part. It is a known route and it works perfectly well; it is only longer than the one that follows it.
The second plan costs time you may not have. It buys a shorter route later, so an apostille from United Kingdom to Vietnam becomes a single stage instead of the first of two. If the paper is wanted before the date, that saving is worth nothing at all, and the longer route is what a real deadline pays for.
The work that both plans share
Nearly everything before the closing desk is identical. Both plans need a record carrying a British public signature, and both send it to The Legalisation Office, which works by post from PO Box 6255, Milton Keynes MK10 1XX and has no counter to visit. Both are charged at £45 / document (standard postal); £40 / document next-day (registered businesses).
So the sensible move is to do the shared work at once and leave the last decision for later. For births, marriages and deaths, Birmingham Register Office at Holliday Street, Birmingham B1 1TJ issues certified copies, so a fresh one can be ordered now, your own record stays at home, and the route is still yours to pick.
Making the call
Put the day the paper is wanted next to 11 September 2026 and compare the two. If your day is earlier, take the consular route and leave room for two desks rather than one. If it is later, the shorter route repays the wait, and the months in between are free preparation time you would have spent anyway.
One caution about the middle ground. A deadline sitting very close to the date is the worst place to gamble, because posts and desks do not run to the day you hoped for. In that position the longer route is the safer one, since it works on both sides of the change and needs no luck at all.