Your travel to Vietnam has to be within the first 60 days.
No one knows what’s happening to the 60 day exemption. The government has said they are changing it back (it was only a temporary measure) but there’s no timeline. Tourism numbers are dropping and they face strong competition from other nations in SE Asia, so they may choose to keep it as it is.
If you can get a job offer with a work permit then you change your visa. The author hasn’t said that he wants to work but I suspect that’s the intention.
It’s unlikely that you can. With 60 day exemption plus 30 day extension then a border run you’ve covered almost 6 months. Keep doing that and you’re likely to be stopped.
Look at DTV. If you can get work online then you can do the ‘digital nomad’ thing