I'm surprised nobody has answered - perhaps because you asked for personal experience and no-one here's done it.

All I can say is I saw signs to transfer desks as I passed through a couple of weeks back. I'm pretty certain that you would not have had to pass through immigration to get to these and that therefore your timings will be fine.

In fact, I've always found immigration OK in Saigon. Even if you did go out and back in, I think you'd be fine - though most nationalities need a visa organised. I have a multientry and couldn't bear not to go out... though unless you were out really quickly it would be a risk, especially if you're not familiar with the place.

Check-in queues can be awful at SGN and can only be escaped if you're in the higher classes or have FF status.